Filling and fairing for GRP moulding

skyflyer

skyflyer

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I have a rather old, cracked and battered instrument pod that houses my engine instruments. It is, I think, moulded plastic - it certainly isn't GRP.

I have brought it home and my intention is to reinforce any cracks on the inside with GRP and then fill and fair any holes and splits and gouges on the outside, polish it up and use the resulting item to make a GRP mould from which I will construct a new GRP instrument pod.

My question is whether ordinary car body filler (isopon, P38, etc) is suitable for the filling and fairing and secondly having done that how polished do i need to get it before applying mould release wax.

I am thinking progressive sanding to maybe 1000 grit wet and dry and then I assume the release wax will put the final gloss finish on it, or should I be looking at finer grit than that? If so - back to question 1 -will the car filler take that degree of fine sanding.

Or - plan B - sand smooth, spray paint with gloss acrylic paint and then apply mould release wax.

Any better ideas?
 
My experience is that pretty much anything will do for GRP plug-making. I've used wood, clay, B&Q own-brand polyfilla, high-build primer, parcel tape, solid wax, polystyrene in a bag... Car body filler sounds ideal.

The sanding and waxing sounds fine - the highest grit I generally have on hand is 320 (for varnishing) and that did the job with several coats of wax on top. How critical the surface is for release depends on the shape of the part, to be fair most of mine have either been flat surfaces or prism-like shapes, which release fairly easily. If you're unsure, blue PVA release agent (not the same thing as PVA glue) will help - it wipes on with a sponge, dries to a plastic film, and then you lay up over it. When it's time to release, get some water in between the layers and the PVA dissolves into a slippery substance so nothing should stick.

Pete
 
As prv says anything can be used. You don't need a super smooth surface for mold release but if you want a gloss surface for the final product then make the plug/mold as well as you can. I tend to use gloss spray and being relatively soft you can get a good surface to make the mold.
So, personally I would fill any faults with soft car filler (I have a lot of the stuff!) shape as required with wet and dry finishing with 800 grade. Spray with any old gloss car paint I have spare and wet and dry any defects with 800 to 1200 grade. Polish with cutting compound, t cut will do. Finish with 10 coats of mold release wax. Ok may be a bit ott but never had a problem releasing complex shapes. PVA release agent is OK for a one off but I do find it can be difficult to release. Water helps to dissolve the PVA. Plug made make the mold. Gelcoat and lay up as normal. To add strength to a big mold I lay old rope and glass it in. The rope adds no real strength but the glass over the top forms a strong ridge. Using the mold I would use wax not PVA as you can then easily make as many products as needed with only a new coat, or two, of wax. I find PVA a bit messy but it's personal taste. Both work.
Also, you may want to use polyester resin rather than epoxy. It's not quite as good and does have some shrinkage, not usually an issue, however, it's more tolerant of inaccurate mixing and wets out the grp better and is a lot cheaper!. If it's not underwater it's more than adequate.
I make the mold in epoxy to keep shrinkage down and product in polyester where the shrinkage helps the release.
You can add colour to the gelcoat, I add it to the grp as well.
Have fun it's not difficult.
 
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I understand how to get to the mould but have one question about the last bit.

When making a locker cover up from the mould how many layers of gelcoat do you use and do you add wax to the first one? ie is the normal process for making up but in reverse?

Mine is a stern locker cover about 900mm by 400mm with rounded corners and 30mm all round wall - if that makes any difference. It's one of those ones that you lift off completely.
 
mattonthesea said:
I understand how to get to the mould but have one question about the last bit.

When making a locker cover up from the mould how many layers of gelcoat do you use and do you add wax to the first one? ie is the normal process for making up but in reverse?

Mine is a stern locker cover about 900mm by 400mm with rounded corners and 30mm all round wall - if that makes any difference. It's one of those ones that you lift off completely.
Click to expand...

I prep the mould with as many coats of wax as I can. Apply like car polish, apply a thin layer and allow to dry then buff off. I do 10 coats on a fresh mould then 1-2 coats per lift. Gel coat is then added, i have only ever used 1 coat. I use a brush most of the time and aim to get about 0.3 to 0.5mm all over, you will find it will be naturally thicker in the corners but thats fine. Its not critical in these applications to be honest as long as there is something there as its just providing a surface finish . I use pigment in the gel and lay up to give an even colour.
 
mattonthesea said:
When making a locker cover up from the mould how many layers of gelcoat do you use and do you add wax to the first one?
Click to expand...

Should only need one layer of gelcoat, though I do use two when I know the mould isn't perfect and the finished article will need a bit of sanding. Makes it less likely that I'll sand through to the mat. Obviously needing to sand the product at all is a bodge...

No wax in gelcoat, that's for flowcoat where it will be exposed to the air during curing.

Pete
 
IMO, post #3 is a very good guide. I would only add a few points :

1) As stated, 10 coats of wax is OTT :)
2) Make the mould thicker than the housing you require.
2) Add stiffeners to the mould as required, to help maintain its shape.

As Pete says, no wax in the gelcoat and i would agree with his use of one or two layers of gelcoat.

The finished article will only be as good as the mould, which will only be as good as the original item, think mirrors :)
 
PaulRainbow said:
IMO, post #3 is a very good guide. I would only add a few points :

1) As stated, 10 coats of wax is OTT :)
2) Make the mould thicker than the housing you require.
2) Add stiffeners to the mould as required, to help maintain its shape.

As Pete says, no wax in the gelcoat and i would agree with his use of one or two layers of gelcoat.

The finished article will only be as good as the mould, which will only be as good as the original item, think mirrors :)
Click to expand...

Good points.
Agree that my use of wax is ott but it doesn't take long and you can't put to many coats on but you can put to few! Having destroyed a complex plug when I couldn't get it to release its a mistake I only want to make once :ambivalence:
 
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