GEL COAT -- What am i doing wrong? now i know!

zorg

ok, over the years i've tried making casts using an initial coat of gel coat but it always turns out bad. (silicone molds)

again i tried this week with a new mold and the layer just pulled away from the silicone and was warped with an uneven surface on the face of the casting. just a reject without a doubt.


i'm guessing
a) the gel coat should not be used on a silicone mold

b)the gel coat thickness was not even

c)im an idiot

d)all of the above

any clues or advice?

thanks

-z

Nwerke

Are you using a mould release? Can you get good results with the gelcoat using a latex or other non-silicone mould (i.e. could the gelcoat itself be off? - also is it old/past its use-by date?)

Are you working in high temperatures? Are you certain you are using the correct amount of catalyst?

Does the piece NEED to be polyester resin? I've never had good luck using polyester resin in silicone; I guess the situation might be different these days with fifteen odd years' advance in chemistry, I dunno, but that was my experience. If the piece is being backed with glass, can you use epoxy instead?

fettster

Hi Chris.

your gel coat is retriculating.

there can be several causes for this, one is over catalising the resin, another is moisure and another is leaving the gel to cure too much before laminating.

if you want, email me at richardmartin4 (@) hotmail.com and we can talk through a solution

Nwerke

Er, reticulating. </smartarse> Fettster is dead right, I forgot to mention that latter point, starting a layup once the gel has kicked is best.

zorg

even after looking it up i still don't know what that means......

e-mail sent


-z

zorg

i don't even get to that point, the gel coat just goes nuts before i get the chance........

Nwerke

Does sound like you're over-catalysing it, gotta say. Or if the weather's hot, that can speed things up too.

If changing to a fresh tin doesn't help, you could try making up your own - mix straight fibreglass resin and car bog until you get it to a gelcoat-like consistency and see if that works. I did a lot of work with that stuff, it can be less of a pain than some gelcoats.

dcc_clone

too much mold release. you should wipe out after spraying. causes gel to "slip" in mold

over catalyzation. too much heat causes thermocracks.

unequal thickness of gel coat. if you have crevices, which gives you more material, will cause the gel coat to pull towards the thicker area during curing. thinning out the surrounding area.

too thin gel coat plus many layers of glass will cause a warp.

moisture. sweat, humidity. messes with chemical properties and causes improper cure.

mold thickness is too thin. heat is pulled away from the layup, will cause orange peel, waviness, improper cure.

fettster

Who's the smartarse? The guy who can't spell or the guy who feels the need to correct him? Lol

ewokus

From the guys who taught me Fiberglassing at Icons (casting up 4 foot ID4 Attack Ships) Gel coat is intended for fiberglass molds only, not RTV. You can try to use Gel-coat, but as you can see, you run into alot of problems, but some people here have used it and got lucky with the results... All the Lost in Space Robot Body parts that were made at Icons, were made with a brushable urethane as the beauty coat, not Gel-coat. BJB and Smooth-on have this brushable urethane ready to go. You don't put it down real thick, just brush in a coat to capture all the details. As it gets tacky, you need to 'dust' the coat of resin with cotton flocking - sprinkle it all around, then use an airhose to remove the excess. (the resin will harden to glass-smooth, and the fiberglass lay-up will not stick to it without the cotton flocking) You now have a great surface for laying up fiberglass resin & cloth to make the casting strong & light. The guy who taught me at Icons has dozens of films under his belt as a model maker - using this exact technique. Hope this helps!

micdavis

Why do you use mold release on silicone? Half the reason and expense of silicone is no release needed. Unless using some exotic casting material.

I'd say you are over catalyzing and waiting too long for the first layer.

stagelife

i think your issue is the mold i dont know of anyone that does a gel coat on silicon molds.... try it on something thats is no silicon and i bet you wont have the issue.... if there is no way around this..... try to dust the mold first with a clear spray paint (try on the outside first to make sure no reaction with old silicon) just my 2 cents

also... you should not really be using mold release on a silicon mold... you dont need it and it could inhibit the cure a bit

zorg

who says i'm using a mold release?

i didn't mention one.

i don't use it.

well i went and tried again with less activator, 1/3 what i normally use and have been using for the last 8 yrs

and........

success!

it came out 98% good, just a couple of air bubbles on some of the corners but overall i'm very happy with the result.

-z

GINO

In my opinion, you should never use a gel coat on a silicone mold unless you're shooting it through a high powered spray gun.
Make sure you use a scale to measure out the accurate amount of catalyst. It's critical.
After, use cabosil to fill in any sharp creases/hard to reach places while the gel coat is still tacky.
Then lastly, start your glass layup using minimal resin.


.

micdavis

LOL. For years I rode a guy because he ALWAYS used too much kicker.

Finally got him to back down on it and he was amazed by the result.

Given time and temperature most stuff will kick with out kicker. A lot of them you can almost just walk it past the stuff and it will go.

Following the labeled instruction works sometimes too.

NEVER use Gel Coat with silicone??!?! Well that's just stupid.

Hangar18Studios

I use gel coats in silicone molds all the time, no problem.
Another important factor to remember is you shouldn't lay up fiberglass in freshly-made silicone molds. Let the silicone cure for a while (a week or more), or bake the silicone after it cures. Silicone emits trace amounts of alcohol upon curing and if you attempt to lay up fiberglass (polyester) in these fresh molds the alcohol will inhibit the cure of your gel coat and make it sticky.

Nwerke

That was my point. </whatever> is faux-HTML for "I'm stopping doing whatever I've just done" - I was having a go at myself for being a spelling Nazi, you see.

I actually am a spelling Nazi, but also had half an idea Zorg might want to look it up. I had to look it up myself when someone first mentioned it to me.

Nwerke

I asked if you were - if you'd said 'yes' I'd have said 'don't'.

Glad you have it sorted! Now post pics.

fettster

Please note the LOL at the end of that post. Lol.

Chris, just got your email( went to my spam folder, will be replying shortly)

clonesix

Has anyone mentioned baby power the molds?

Yeah, gelcoating silicone molds (especially new ones) is tough. The gelcoat doesn't have anything to grab onto.

I have had good luck with baby powder, and spraying the gelcoat.

zorg

fettster did tell me about the baby powder trick ,although i had heard of it before i thought it only applied to runny resin rather than this gel stuff.

as for spraying, i have not tried it, how do you go about it?

i would have thought it would be too thick to go through a spraygun?

-z

foxbatkllr

Spraying is nice, but the spray guns are expensive and they don't work for every project. Some molds are difficult to spray into because of their shape. Honestly, I've had good success brushing gelcoat. Gelcoat can have a problem sticking to silicone sometimes when you brush it on, but to say you should never brush it into a silicone mold is ridiculous. It's manageable and its much better than having no gelcoat IMO.

zorg

well i have been wiping down the surface of the mold with acetone and the gelcoat seems to brush on fine, almost no air bubbles now.

i think for the amount of times i'll need to spray gel coat its not work the outlay.

a lot of the things i do would not be suitable for spraying anyway.

-z

exoray

You can get by with a $15 Harbor Freight spray gun with the tip drilled out to give or take 2mm... I hear that you can actually contact Harbor Freight customer service and they will order you up the larger tips, IMO it's easier to just drill them out, you are not really overly concerned with a nice spray pattern, so the hack job with the drill will work...

20 Oz. High Volume Low Pressure Gravity Feed Spray Gun

Hardly a perfect gun but it will get the job done, some people even use cheap engine cleaning chemical sprayers...

Amazon.com: AIR ENGINE CLEANING GUN WITH WAND & HOSE: Automotive

These certainly are not the ideal tools for the job, but they will get the job done for the casual hobbiest that doesn't want to brush, and also wants to keep the outlay minimal...

Cleanup on these guns is not always the easiest but a 5 gallon bucket with a gallon or two of acetone in it and a suspended screen bottom will help with cleanup...

I personally gave up on polyester gel coats, in favor of a proprietary urethane blend that I worked up after much trial and error... As said above though the polyester layup doesn't readily bond to urethane in general, so you need to work out that issue... The above cotton flocking is one way there are others... I just find the urethane to be more predictable for my needs...

Also what you really should do when you are new to fiber-glassing is keep logs, get a humidity and temperature gauge and keep detailed logs of humidity, temperature, catalyst ratio, work time and cure time and your overall opinion of the end results good and bad... As your log and experience grows you will discover the sweet spots that work... You will also discover real fast just how much havoc humidity and temperature can play with polyester, and how important it is to adjust the catalyst for varying conditions... In time you will just get the knack for it, but the logs are always good reference...

womo68

Great thread. I have a question....I have molds (silicone) and every time I use gel-coat it seems to crack a little as if it shrinks. I have to go back and fix the areas with bondo. I'm assuming when it kicks it shrinks. I also slush it around until it kicks....so no brushing it on.

So question is....Is there a way to keep the gelcoat from pulling away from the molds?