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How to repair a GRP on a RIB

Overview

Introduction to repairing GRPPrincipal Products RequiredChoice of Hardener Filler IngredientsPreparation Guideline for Filling on Different SubstratesCobination !uantities of Resin and Filler IngredientsOrdering Resin S"ste and Fillers#easuring out $po%" Resin and Hardener b" &eight'sing $po%" Filler in FairingCuring

&or(ing with Cured $po%" Filler 

Introduction to repairing GRP

Most craft of whatever size or construction material have required filling and fairing compounds atsome time either in their life or in their development and design. Production GRP hulls which emergefrom the moulds with a smooth gel coat finish owe their fairness and quality of finish to the skill of thebuilders original shape or 'plug' from which the mould was formed.

he same high standard of finish can also be seen in 'one!off' composite racing sailboats. hese craftmostly use epo"y rather than polyester resin in their construction and are built over a male plug. #llone!off racing sailboats use lightweight epo"y filler mi"ers and specialized high performance paint

schemes to obtain the necessary high standard of finish.

Recently we have seen an increase in the number of craft$ both mono and multihulls$ which use foamor wood as the principal core materials in their construction$ particularly P%& (#) foam or cedarstrip. hese hulls rely on e"tensive filling and fairing to give a surface finish which can match that ofmoulded GRP hulls. (teel boatbuilders also use epo"y filler to fair their hulls once the shaped steelplates have been welded together. *iller will take out any slight curvature imperfections and hide thewelding lines before a final paint system is applied.

*illing and fairing is a very labour intensive operation but an essential one necessary to ensure thatthe hull offers the least resistance to movement through the water. )owhere is efficient filling andfairing more important than on underwater appendages ! the keel and rudder$ where not only sectionshape can be controlled by application of filler$ but surface finish$ fairness and smoothness can beperfected for noticeable performance gains.

his guide outlines the standard procedures which are necessary in order to achieve the best qualityfinish on wood$ composite$ ferrocement and metal surfaces.

Principal Products RequiredResin S"ste

 # resin system of some type$ usually epo"y or polyester$ forms the basis of every filler$ together with ami"ture of inert filler ingredients. hrough the action of a catalyst$ in the case of polyester$ the resin is

caused to polymerise to a hard plastic. +ith epo"y based products$ the resin reacts with a specialisedhardener and together they 'crosslink' and polymerise to form an outwardly similar looking hard plasticmaterial.

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*or marine use especially$ epo"y resin based fillers are far superior to those based on polyester resinas they are inherently more water resistant$ shrink less during curing$ are more adhesive and have ahigher degree of toughness and strength. #lthough epo"y!based fillers cost more they are the onlytype recommended for boat hulls. he epo"y resin and hardener can be supplied with filler additionspre!mi"ed into one or both components as a 'formulated' filler$ but most professional users working ona large scale prefer to create their own mi" by using a liquid epo"y resin system and low density filler

'powders'. his gives the user considerably more control over application and working properties overa wide range of working conditions. he user can vary the quantity of filler added to create 'thick' or'thin' mi"es. he resin system will have a number ,usually at least - of different hardeners$ each ofwhich will offer different levels of reactivity when mi"ed with the resin component. his approach isreferred to as the /recipe/ method.

Resin systems have roughly the same working properties ,pot!life$ gel time etc. when used with theirfast hardeners. he gel times of systems using fast hardeners range from about 01!02 minutes.3owever$ the term 'slow' for a hardener is only relative$ such that the gel time for one system's slowhardener may be much longer than another system's slow hardener ,e.g. from 04 minutes to -51minutes. he reader is advised to consult the data sheets of the products and note the 'gel times' ofthe proposed hardener and resin mi"es$ as this will give a relative inde" of working time of the finalresinhardenerfiller mi"es.

Choice of Hardener 

 #ppropriate selection of hardener can give mi"es with different degrees of reactivity or speed. *inalchoice of resin system and speed of hardener will largely depend on size of mi" required$ ambienttemperature$ nature of substrate and total thickness of filler required. 6t is worth noting that it ispossible to blend hardeners from the same system to produce intermediate gel times.

7ften$ it is possible to use the same resin system that is being used to build the component. hissaves the inconvenience of buying a second system but it may not be the most cost!effectiveapproach$ as another system maybe cheaper.

Filler Ingredients

+hereas the resin system will control the working characteristics$ mechanical and chemical propertiesof a filler$ the filler ingredients will dictate how easy the filler will be to apply and finally sand down toprovide a fair smooth finish. he 'hollow sphere' type of filler is the only type which will create a fillermi" with the two desirable characteristics of being both lightweight and relatively easy to sand. histype of filler can also be used for structural adhesive purposes$ especially for low density filletbonding. 3owever$ the glue mi" created is relatively weak and therefore it should be restricted tocertain types of wood bonding. 3ollow spheres are often described as /microspheres/ and are veryeffective at reducing the density of the mi" by displacing resin. #s the microspheres have a hard$ resinresistant 'shell' the resin does not penetrate the 'interior' of the bubble. #lthough the user is free to add

whatever filler proportion serves his particular needs$ there is a ma"imum practical additive level foreach type of filler powder. here must always be sufficient resin present to coat each microsphereand bond one microsphere to the ne"t$ effectively creating a type of 'foam'.

he greater the proportion of the filler ingredient added$ the lower the density of the filler mi" willbecome. 6f too much filler powder is added to the resin ,beyond the point where all microspheres areeffectively coated$ the mi" will become 'dry' and 'pasty'$ and difficult to work. 6t also may not bond wellto the surface to which it is being applied.

he two principal types of hollow bubble filler are phenolic microballoons and glass bubbles. able 0compares the characteristics of filler mi"es.

Table 1. Comparative Properties of Fairing Mixes Created from Different "Hollow Sphere"Ingredients

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Property Ranking rder !

Density !#$ightest #Heaviest Phenoli% Mi%ro&alloons 'lass ()&&lesPhysi%al Hardness !#Softest #Hardest Phenoli% Mi%ro&alloons 'lass ()&&les

*ase of Sanding !#*asiest #Diffi%)lt Phenoli% Mi%ro&alloons 'lass ()&&les

+ater Resistan%e !#Highest #$owest 'lass ()&&les Phenoli% Mi%ro&alloonsCost !#Cheap #*xpensive 'lass ()&&les Phenoli% Mi%ro&alloons

Preparation Guideline for Filling on DifferentSubstrates&ood

+ood is an absorbent material and therefore the best possible adhesion is obtained by first pre!coating with an unfilled solvent free epo"y resin and hardener mi". he filler mi" can then be appliedeither after first leaving the surface to cure and then sanding to obtain a good key$ or whilst the initialunfilled epo"y coating is still wet or tacky. 8eep indentations or screw holes may be filled without pre!coating.

Pol"ester GRPa) Fibrous *ainate Surface without Gel Coat

Solvent wipe with a fast evaporating epoxy solvent, Pre-%oat with %lear. )nfilled solvent-free

epoxy resin mix first and treat as for &are wood, Polyester 'RP sho)ld &e well %)red

otherwise the epoxy %)re will &e inhi&ited and there will &e poor adhesion to the s)rfa%e,

b) Pol"ester Gelcoat Surface

+et sand with 021 ! 091 grit paper then solvent wipe with a fast evaporating epo"y solvent to obtain agood key. he polyester should be well!cured.

Pol"ester Filler or $po%" Filler 

 # previously filled surface should be well abraded with :1 ! 91 grit production paper and wiped withfast evaporating solvent before commencing any further filling. he polyester should be well!cured ifepo"y filler is to be used.

$po%" FRP

his may or may not have used nylon peel ply incorporated into the last laminate layer. 6f it had$ thentear away the peel ply and simply apply the filler. 6f no peel ply was used then sand the surfacethoroughly with 91 grit production paper and clean with a fast evaporating solvent such as cleanacetone.

Cobination !uantities of Resin and FillerIngredients

+here the 'recipe' method of creating an epo"y filler mi" has been selected$ the following informationcontains guidelines for making the mi" to a fairing consistency. he relative quantities may be variedby the user to create thinner$ thicker or more thi"otropic mi"es to suit individual application needs.

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Mi"es of lower density can be created for other applications e.g. to create a low density 'syntactic'foam for filling rather than fairing purposes. 3ere the mi" may have a higher filler loading to allowapplication by trowel in specific areas. #s a specific e"ample a Microballoon mi" for this purpose canbe made with a density of 1.2 gcc rather than one nearer 1.; gcc used for fairing. *airing mi"es arerequired to be more mobile because of the greater area to be covered and to suit the tools andapplication equipment used. )ote the inclusion of colloidal silica to the mi" in every case.

 #lthough added in relatively small volume compared to the primary filler ingredient$ colloidal silicaserves the very useful function of giving additional non!sag properties to the mi". his is a particularlydesirable for vertical or sloping surfaces where some 'sagging' in a filler mi" may be e"pected whilstthe filler is still in a workable state$ before sufficient hardening has taken place. he following materialweights ,able - will create one litre of low density epo"y filler.

Table 2. +eights to %reate ! litre of low density epoxy filler 

Primary Filler Primary Filler +eight Colloidal Sili%a +eightResin / Hardener

+eight0otal Filler Mix

+eight1pprox, Density of

Mix

Mi%ro&alloons !2g !!g 324g 5264g 4,7g8%%

'lass ()&&les !2g !9g 994g 53:4' 4,2g8%%

7ne litre of filler$ if spread evenly$ will produce an average 0mm thickness over an area of a squaremetre. Generally$ for filling and fairing on rough surfaces$ sufficient quantities of resin system andfillers should be ordered to allow for an average thickness of -mm ,- litres of filler mi" per sq.m. hiswill allow for uneven surfaces and losses through sanding.

Ordering Resin S"ste and Fillers

he following information will be useful when ordering suitable epo"y resin system and filleringredients to create fairing mi"es using Microballoons. he quantities indicated in able 5 willproduce a low density filler mi" sufficient to cover the area shown to an average thickness of 0.1mm.

Table 3. Material ;)antities re;)ired to prod)%e a low density filler mix s)ffi%ient to %over

the area shown to an average thi%kness of !,4mm,

1rea +t of Resin < Hardener =kg> +t of Mi%ro&alloons =kg> +t of Colloidal Sili%a =kg>

2 , 4,7: 4,47!4 3,3 !,99 4,!!

4 6,6 ,77 4,

94 !9, 9,?? 4,99

34 !:,7 2,9 4,3324 ,4 7,72 4,22

#easuring out $po%" Resin and Hardener b" &eight

he weight method of measuring relative amounts of epo"y resin and hardener is the most accuratemethod of combining the components in e"actly the right ratio. %olume methods using graduated cupsmay be sufficiently accurate for epo"y systems with simple mi" ratios such as -<0 or 5<0 resin!to!hardener ratios. Metering resin and hardener using pumps may be appropriate for small volumesfound in some adhesive applications but is too laborious for the volumes generally used for creatingfairing mi"es.

Measuring by weight is easily achieved using simple$ ine"pensive electronic scales. hose of either -kg or 2 kg capacity which have an accuracy of = -> are most suitable.

'sing $po%" Filler in FairingProcedures

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his section is divided into<

a> Overall Fairing + the removal of high spots in order to %reate a fair shape whi%h &lends

 perfe%tly into the overall shape,

 &> *ocali,ed Fairing + making small areas s)%h as fastening holes or lo%al voids smooth tothe to)%h and )ndete%ta&le,

Preparing the #i%

(elect an appropriate epo"y resin system and hardener speed to suit the application. Measure outsufficient quantities of materials to make a suitable quantity of filler which can be spread over the workarea and made smooth well within the working time that the hardener will allow. 6deally start with smallvolumes until some idea of working time is gained. 6n order to prevent early gelling of the resin andhardener before the application is finished$ reduce mi" volumes if ambient temperature levels rise or afaster hardener is used.

Mi" components thoroughly. #lways mi" resin and hardener thoroughly first before adding filleringredients. &hoose a suitable vessel such as a smooth!sided plastic tub in which to mi" the filler.7nce mi"ed$ transfer the filler mi" to a board and spread thinly ,maybe up to 01 mm thick to helpdissipate the heat of the resinhardener chemical reaction and so lengthen the working time of themi".

-pplicationa) Overall Fairing

?sing a plasterers 'hawk' to hold a suitable volume of filler$ transfer a manageable amount to the

surface using a fle"ible metal applicator and apply in thin layers to avoid air pockets. *or fairing largeareas of 5!2m- at a time on boat hulls use a plastic fairing batten ,a length of plastic drain pipe issometimes used held by two people.

 #im at a slight overfill. #pply no more than 5!: mm at a time. 7n very unfair hulls the 'notchedapplicator' technique is useful whereby the initial filler application is by a batten with a 'notched' orserrated edge$ which gives a furrowed surface to the filler. 7nce hardened the surface is sandedusing long sanding boards which will take off the tops of the 'ridges' on the high areas and leave themintact on the low areas of the surface. his technique ensures that the minimum of filler is used andsanding time is shortened because only 21> of the normal filler quantity is removed with everysanding stroke.

7nce fair the hollow areas and 'furrows' are filled level to the original epo"y surface$ it is useful to use

a slightly colour!tinted mi" for the second filler application to differentiate it from the first whensubsequently sanding.

b) *ocali,ed Fairing

 #pply the filler with a spatula or metal straight edge. # higher loading of primary filler is often usedtogether with colloidal silica$ to create a stiffer filler mi" for this type of filling.

Curing

 #ll epo"y filler mi"es will sand most easily when fully cured because a fully polymerised epo"y will notsoften as a result of the heat generating abrasive action. (ince most filler mi"es are sanded beforebeing fully cured$ a coarser grade of paper will be necessary in order to avoid 'clogging'. +arm

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conditions with$ ideally$ low humidity help the epo"y components to react most effectively. Please notethat when sanding non fully cured epo"y care should be taken to ensure minimal skin e"posure to thesanding dust and that dust masks should be warn. Please see each resin system's instruction sheetfor details.

&or(ing with Cured $po%" Filler Preliinar" Fairing

he aim of fairing is to remove e"cess filler in a controlled manner in order to ensure a visually'perfect' surface. # variety of tools and cutting edges may be used in the initial stages to remove theworst lumps of filler to create a surface on which abrasive paper will be most effective. he following,able : outlines some of the tools which may be used.

Table 4. 0ools )sed in fairing

S)rfa%e 0ool +orking Mode

Coarse l)mps < &)mps @S)rform@

Ca&inet S%raper 

M)ltiple (lade

Single (lade

  1ngle 'rinder 1&rasive sanding pad and dis%

Smooth Filler Stiff or Flexi&le (oard

*le%tri% or air powered random or&ital sander 

1&rasive Paper sheet or a&rasive dis%,

'sing -brasive Papers in Fairing and Finishing

wo basic types of abrasive paper are usually used. *or coarse sanding the paper used is aluminiumo"ide 'production' paper obtained by the metre from a roll. his can be cut to convenient length to useon boards or mechanical sanders. he other type is wet or dry silicon carbide paper which can beused either wet ,with water as the 'lubricant'$ or dry and is always used for final finishing of coatings.he abrasive paper is usually cut and used on a rubber palm!held sanding pad. he following ,able

2 is a guide to the abrasive grades used at the different stages of fairing.

Table 5. 'rade and 0ype of 1&rasive Paper 

S)rfa%e 0ype of Sanding Paper 'rade +et-or-Dry 'rade

*poxy Filler or +ood 'eneral Ro)gh FairingFine Fairing

Finishing =wood only>

34 - 7464 - !44

!4 - !64

 Aot BsedRarely Bsed

!64 - 4

Polyester 'el%oat S)rfa%e Preparation !4 - !64 !64

High-&)ild *poxy S)rfa%er 8Bnder%oat

Fine S)rfa%e Fairing 64 - !4 !4 - !64

Poly)rethane )nder%oats ery Fine S)rfa%e Finish

eying for Finish Coats

!64

 Aot Bsed

!64 - 4

64 - 94

Poly)rethane Finish Coats ery Fine S)rfa%e Fairing and

eying

 Aot )sed 64 - 94

)ote< he lower the 'grade' number the larger the grit particle and the lowest density of cuttingparticles.

Fine Fairing and Finishing

7nce the main fairing has been completed$ leaving a sanded$ fair$ smooth surface$ the ne"t stage isto use a type of coating system which is of fine filler ingredients. hese high build$ undercoatsurfacertype of products are more common in the automotive industry but are becoming increasinglyusedcharacterised by its high loading as part of the fairing process in the marine industry.

.ac( to /op

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