Making a mould for fibreglass - RC Groups
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Mar 05, 2012, 03:27 AM
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Making a mould for fibreglass

For several years now I've been making moulds to make fibreglass components for various parts of my aircraft. I make them out of balsa usually, then after rough sanding them I apply car body filler and sand until smooth. I then apply a spray coat of car body primer paint, but then the problems start. The grain of the wood swells, so I have to then go over the sanding process again, then more primer, then more sanding, until I get tired of it and apply a coat of finishing paint. The resulting fibreglass part, or mould, still shows signs of wood grain, but I then sand these out.

But now I'm doing a more ambitious mould -- the complete 40"-long fuselage for an F16 -- and I'd like to have a grain-free result first time round. This mould is balsa infill between sections which have been cut from 3mm liteply. Filling and sanding is progressing well, and I'm now trying to figure what's the best way to finish it off. Is there any particular material that would (a) minimise the amount of swelling and (b) harden any exposed wood so that, once sanded down, it won't swell again with the second coat? Instead of automotive paint, I've been wondering if polyester finishing resin, or water-based polyurethane paint, would be good.

I've already experimented on a small separate piece applying release wax direct to the wood/filler mould surface, and then pva release agent, but the pva swelled the wood.
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Mar 05, 2012, 04:58 AM
Crash proof pilot
FlyDave's Avatar
two things you can try, if you haven't already tried :

proposal 1 : apply a wood sealer. I don't know the right english term, but is a thing "like a paint" that is used to fill the pores of the wood before painting it. You can find it on every shop that sell paints.

proposal 2 : apply a coat or two of slow cure epoxy resin on the whole surface, then sand it smooth. You obtain a sealed wood surface.
Mar 05, 2012, 05:16 AM
An itch?. Scratch build.
eflightray's Avatar
If possible I prefer to use foam for any fiber glassing work, mainly because I have had similar problems using balsa, especially where the end grain comes to the surface.

I use a very 'fine grain' blue foam for the plugs, it sands quite well. I then use a matt ceiling emulsion as a 'sealer' and as a good sandable primer. Any water based product tend to make balsa even more 'hairy'.

The - Composites Fabrication forum - is very good for finding what others have used.

I have never tried a full fuselage, good luck with the project. Thread ?
Mar 05, 2012, 06:00 AM
Registered User
I've got a thread going here, eflightray.

And thanks for the link to the Composites Fabrication forum -- I hadn't realised it was there as a separate topic.

Where do you get blue foam from, and how do you define your sections without using ply (or similar) slices?

FlyDave, I think my primer is what you'd call a sealer though, admittedly, it's not designed for wood. I'll try some wood primer on a sample and see how that goes.
Mar 05, 2012, 08:01 AM
An itch?. Scratch build.
eflightray's Avatar
My better quality blue foam, (which I have recently run out of), was called 'Floormate'. It came in 2" thick, and something like 8ft x 2ft sheets.

I bought mine from a builders insulation stockists back in Coventry.

I never made anything as big as fuselage, but see no reason why you couldn't use temporary thin other material for a former for any hot wire cutting, in smaller sections where possible. Then discard the former and butt join the sections together for final shaping and sanding.

I did make some 'reverse profile' formers once, (the remaining material after the former has been cut out), for checking the outside shape of a section. But then I still tend to treat 'scale' as "it looks about right", rather than keeping to accurate profiles.
Mar 05, 2012, 08:34 AM
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Thanks. I'm not planning to be super scale, but "looks about right" including details such as gun ports and other various bulges is what I want -- and no wood grain.

Anyway, this afternoon I'm going to make up a balsa/liteply test block and try some wood primer, some wbpu, and some finishing resin on it.
Mar 05, 2012, 08:42 AM
Registered User
I usually coat the plug with epoxy or polyester resin (on wood only, not foam ), sometimes over tissue, then sand that for a good clean surface. I also prefer to make plugs from softwood or foam rather than balsa as either of those have better surfaces in the first place.

I started using resin because I always had some around on account of that's what I'll be using for the layup. But I've sometimes used out-of-date resin for the plug that I wouldn't really trust for the main layup .

Mar 05, 2012, 03:38 PM
Crash proof pilot
FlyDave's Avatar
Originally Posted by abenn
Anyway, this afternoon I'm going to make up a balsa/liteply test block and try some wood primer, some wbpu, and some finishing resin on it.
Be careful if you plan to use wbpu directly on wood because the water will swell the balsa and you risk to ruin the plug.

This is a product you can find on UK, to fill the pores before the finish :

(surely you can find many other similar product, but as i don't live on uk i cannot find the right term)
Mar 06, 2012, 03:22 PM
Registered User

Test results!

OK, I've done a quick and basic test. A sandwich of balsa and liteply, sanded smooth, then a layer of auto body filler applied and sanded down, then three strips of different "paints" -- water-based polyurethane, water based wood primer/undercoat, and polyester finishing resin. After leaving it overnight for the resin to harden, I then sanded it smooth using my Permagrit fine sander, then a 3M "180 Fine" sanding block. Lastly I gave it a quick spray coat of auto primer paint, and sanded that with my 3M block.

My initial observation is that none of the paints had caused any significant swelling of the balsa -- probably because there was an extremely thin layer of body filler covering most of the surface. Secondly, the resin cured with "dimples" in its surface, and was so hard that sanding it completely smooth was a chore that I'd rather avoid. My opinion is that the wood primer/undercoat paint was the best solution -- it was easy to sand smooth and, after a second coat and sanding, took a thin spray coat of auto primer which sanded down to what I think is a very good finish.

I'm still not sure if I need a finishing coat on top of the primer before I start applying mould-release wax. I'll try a few coats of wax straight onto the grey primer, and see if the pva release agent then causes any grain to show. I'm guessing it's a case of having to put on many more coats of wax than I usually do (one!), so that it will be impervious to whatever goes on top.
Mar 14, 2012, 06:58 AM
Registered User
Originally Posted by eflightray
My better quality blue foam, (which I have recently run out of), was called 'Floormate'. It came in 2" thick, and something like 8ft x 2ft sheets. ....
Any idea if Jablite is equivalent? It seems to be the most readily available one in small quantities -- I've seen Floormate for sale at 82 for a pack of eight sheets, but B&Q do Jablite much cheaper, and single sheets.
Mar 14, 2012, 09:41 AM
Registered User
If that's the stuff I'm thinking of it's a lightweight open-cell expanded polystyrene (EPS) foam and almost impossible to get a decent finish on.

The good stuff is closed-cell extruded polystyrene (XPS) foam. If you need some relatively small bits I've got a fair amount of 80mm Floormate I deliberately over-ordered when I was insulating my garage/workshop (drop me a PM). Alternatively you can buy it from Balsamart in smallish amounts for a lot less than 82 quid

Mar 14, 2012, 01:31 PM
Registered User
Thanks for the info, and the offer, Steve. I didn't know the terms EPS and XPS but, now that I do, I'm better equipped to search for the right thing next time I go to B&Q. According to a thread on another site, they stock some kind of XPS.
Mar 14, 2012, 02:05 PM
Registered User
I use PU foam, carve and sand easy, apply grey primer, sand apply again, sand polish, release lay up.
Mar 15, 2012, 08:33 PM
It must have a machinegun
Scaledown's Avatar
I've been making plugs from balsa for vac forming canopies and I've always managed to get a good finish.
Firstly I try to get the best surface I can from the bare balsa, finishing with 400 grit paper.
Dents or joints are filled with dope/talcum mix and sanded back.
Then I give it a coat of dope to seal it.
Then I spray it with high build automotive primer. It needs a few coats to fill the grain. Then I sand it smooth.
On a small part I will use dope/talcum mix spread on with a knife in lieu of the spray primer.
Mar 15, 2012, 09:08 PM
Registered User
Best way to seal a wooden plug in order for it to last... FIBERGLASS IT!

No point in being cheap in not putting a layer or 1 or 2oz glass cloth over the plug to give a nice dent resistant surface, that will Also seal it to keep it from any moisture from ever reaching the wood and causing it to shrink or swell.

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