Another mold question...

...this time fiberglass. I'm probably going to make a fiberglass bonnet and boot lid or the Jensenator at some point, I plan to do this by pulling molds from the existing pieces. So I get the basic technique but these are gonna be BIG pieces and I don't want to screw things up any worse than I absolutely have to. I want to be able to make two bonnets and two boot lids from these molds, always nice to have a spare and maybe be able to modify one for scoops etc.

The plan is to sand then paint the existing panels merely to get rid of the stripes and stuff that are wrinkly, use cheap paint. So then that means I have to coat these painted plugs with a release agent. So far so good. Given the size of the panels, there are two things I need to address: a really good mold release and adding release system parts.

I read awhile back it's possible to use metal tire valve stems strategically placed, the idea is to coat the crap out of the original part, start laying up the glass for the mold, place the valve stems while laying that up, adding some bracing (typically EMT conduit) so it will help hold its shape, then when hard shoot compressed air through the valves to help separate it from the original piece.

Any experience out there?

Unfortunately, no fibreglass experience here, but I did stay in a holiday inn... No actually, I did work in an injection mold making shop, and while I was there we made a couple of molds for largish slightly contoured parts and we used poppet valves like these to get the part to release in a timely manner. I can't see why it wouldn't work for making a low use mold as well.

On something as relatively flat as that you wont need to go through all that. A good set of wedges and an air wedge will get you there (http://www.fibreglast.com/product/Air_Injected_Wedge_00958_A/Mold_Releases)

Make your part strong enough to resist the forces and you'll be able to work it off there. I've done at least 3 or 4 hoods.

Be careful of using a cheap paint to coat. Depending on your resin it can/will react with your gelcoat and cause voids in the gelcoat. These can be repaired but it takes effort. If you can find my old Yugo build thread where I was taking a mold of the hood you can see the steps to molding a hood and what I had to do to repair gelcoat voids. Feel free to shoot me an email with questions. I've got a few glass jobs coming up, Volvo hood and it looks like I might be doing some body repairs on a Reliant Robin very soon.

If you're only going to use it for a few production parts, then make the mold with plaster instead of fiberglass. Google it up. My factoid database says:

Use plaster and hemp fabric from the fabric store to make a mold. Use only Hydrocal B-11 plaster. Johnson paste wax + PVA for Release Agent.

"They Say" it will work fine for several parts.

Hm. Hadn't though about that. Original plan was to do it this way:

https://www.youtube.com/watch?v=BNFz6nxBlNU

https://www.youtube.com/watch?v=UZoVrnY8EIo

https://www.youtube.com/watch?v=alUm4QzRQOA

then make two each from the mold.

I'd advise against brushing on the gelcoat as shown in those videos. It was advice I didn't heed and it led to a lot more work. You can brush on gelcoat but it takes experience and practice to get it right. If you decide to brush it on then go heavy with it.

Yeah, I thought it might be going on a bit thin and that's one reason I'm asking here; practical advice. I thought I might get a cheap spray gun for shooting the gel then lay up the 'glass the way they show, dunno if a cheap gun would help with shooting resin, my gut instinct is no.

I saw another one where the guy just laid 'glass over the existing hood, then finished the outside of that part and used it, i.e. he didn't make a mold. He mentioned the dimensions will be off (too large). It was for a drag car, all he was after was weight reduction.

I've done it that way many times as well, the hood for the Wartburg was done in the moldless method. It takes a lot of sanding to get a nice product, if you want more than one pull a mold. The key with the moldless, to keep the dimensions close, is to only lay up 2-3 layers max then pop it off and reinforce it from the underside with more layers.

Depending on how soon you want to attempt this I could loan you my air wedge and gelcoat gun as soon as I finish the Volvo hood I need to make but that may be a couple of months. You can brush resin no problem, but the gelcoat really should be sprayed.

just remember.. gel coat flexes less than the glass itself. Do not go too heavy on it or you will eventually get crazing

mad_machine wrote: just remember.. gel coat flexes less than the glass itself. Do not go too heavy on it or you will eventually get crazing

Which is why it's better to spray than brush, because it's very difficult to get an even, complete, coating with a brush without going heavy.

On crazing: the molded part will need a gel coat as well, i.e. once the mold pops off then it's coated with release agent, then gel coat, then fiberglass and reinforcement as needed. The vids mention 6 mils thick which isn't much at all; the final piece will need some sanding of course and I don't see 6 mils as being enough buffer for my goofy clumsy self.

That brings the question: how to repair the gel coat on the finished part, if needed? Or can it be painted over without a big deal? Will paint directly on the fiberglass without gel coat cause delamination? Keep in mind this is lightweight race car fiberglass, not a show car.

Curmudgeon wrote: On crazing: the molded part will need a gel coat as well, i.e. once the mold pops off then it's coated with release agent, then gel coat, then fiberglass and reinforcement as needed. The vids mention 6 mils thick which isn't much at all; the final piece will need some sanding of course and I don't see 6 mils as being enough buffer for my goofy clumsy self. That brings the question: how to repair the gel coat on the finished part, if needed? Or can it be painted over without a big deal? Will paint directly on the fiberglass without gel coat cause delamination? Keep in mind this is lightweight race car fiberglass, not a show car.

You can re-gel a part same as re-painting it. You need to sand the repair/recoat area and clean it well, then reapply. Of course it will need sanded smooth. The gelcoat I use contains a bit of wax which rises to the surface to seal it from the air while it cures. If a 2nd coat or repair is required then that wax layer needs to be removed (its not really visible)

Short answer, yes you can re gelcoat, or with a proper sealer primer you can just paint the whole thing without recoating.

Good to know.

Next stupid question: for me this will have to happen in the off season. I don't have access to a huge heated shop, I will probably have to pull the 'plugs' (OE panels) from the car, store the car elsewhere for a week or two as I make these parts.

So what effect will temperatures have? I do know that laying up fiberglass in the open is not a real good idea (I've made a few small parts and done some repairs with it) due to UV radiation. So just how critical is temperature during this process with large parts? The time of year I'm thinking would probably involve lows in the 20's and highs in the 50's. Will that furschnorkel (VW service term! ) things badly?

Oh yeah, about paint: my idea was to 600 grit or so the 'plugs', then use foam rollers and tractor paint or similar.

Depends on what type of resin you use. Actually I'll back up and say, in those temps, you'll use epoxy resin. I use polyester resin so this is where my knowledge parts from your needs other than to say epoxy is actually better, but poly is a bit easier to work with (or just what I've worked with forever so I know it better) but poly cures through heat, below certain temperatures you either wont get a cure or will be adding so much MEK activator that it will cure up too quick.

I do not think epoxy resin has this issue but I dont know.

Hmmm. I better research curing temps and see if I can maybe warm the place up somehow during the cure. I have (on small pieces and repairs again) bumped the MEK up a little for cold temps. Not sure on big stuff like this, though.

Stupid question: what about IR lamps etc to help with the curing? Never tried that on big stuff. I did make a small transmission tunnel extension on a cold day once (IIRC upper 40's), I laid wire mesh for the shape, got it riveted into place, coated it with resin (no gel coat, it was just a rough part hidden by carpet), laid and pressed my glass cloth on that, pressed more resin into the glass cloth then heated it with a blow dryer till the resin began to set and was tacky on the surface, after that I just left it alone. Next morning it was hard and the part worked fine. Of course, disclaimer again, this part was probably about 12x18" by 3/16" thick max.

Curmudgeon wrote: Hmmm. I better research curing temps and see if I can maybe warm the place up somehow during the cure. I have (on small pieces and repairs again) bumped the MEK up a little for cold temps. Not sure on big stuff like this, though. Stupid question: what about IR lamps etc to help with the curing? Never tried that on big stuff. I did make a small transmission tunnel extension on a cold day once (IIRC upper 40's), I laid wire mesh for the shape, got it riveted into place, coated it with resin (no gel coat, it was just a rough part hidden by carpet), laid and pressed my glass cloth on that, pressed more resin into the glass cloth then heated it with a blow dryer till the resin began to set and was tacky on the surface, after that I just left it alone. Next morning it was hard and the part worked fine. Of course, disclaimer again, this part was probably about 12x18" by 3/16" thick max.

I've used heat guns in the past to cure a part, I would suspect the same could be done with an IR lamp.

Just to be clear, are you planning on gel coating the inside of the mold? I ask because I have never done that, only on the finished parts and not even then if I was going for light weight. For everything I have made I went with a couple extra coats of high build primer instead of the hassle of gel coat.

Yes you should gel coat the inside of a mold, it is a better surface for release agents than raw fiberglass.

High build primer or gel coat, same process as far as spray to apply so really not much more effort but the gel coat is a much more durable surface layer.

That's the plan: gel coat the 'plug' (i.e. the bonnet and boot lids) and then lift the molds from them. Then use the molds to make the actual part.

It seems there is a split out there on whether gel coat is necessary on a low production part which will require some finishing. That has me wondering whether to gel coat the plug (i.e. my original metal bonnet and boot) heavily, build up and then pull the mold, mold release the crap out of the mold's gel coat then put resin and mat directly on that, i.e. don't shoot gel coat inside the mold for the finished part. The part (as opposed to the mold) will have to be sealed and painted to match the rest of the car anyway, is my thinking.

If I'm wrong it certainly won't be the first time so sound off!

You can do it the way you describe, but if you have the gel coat already why not use it? It's just going to provide a better surface for final finishing and help prevent sanding into the fibers during your finish sanding.

I've made plenty of non coated parts though, works fine, just seems to add to the effort of finishing them.

the place I used to work that did a lot of hand crafted FRP parts used to use wax paper, packaging tape & tin foil on items that they molded. I did that when I made a part - used tin foil and packing tape and then waxed it. That allowed it to pop off.

I saw that method on another video too. In fact IIRC the V12 MGB in Projects had the rear section of the body made that way.