to gelcoat or to use duratec...advice needed

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#1

I’ve just about had it with using Duratec as an in-mold substitute for a gelcoat. I’ll describe my problems with Duratec, but if I try a gelcoat, which one?

I’ve used just about every spraygun on the face of the earth. $15 Harbor Freight guns, $100 hvlp guns with 2mm tips, cheap Chinese gravity guns with 2mm tips…and I get the same inconsistant results.

I’m spraying into fairly large car body sized molds, so it takes a lot. I’ve found it can be sprayed fairly easily before setting up in the gun (it happens) if I mix up the Duratec and then use a wee bit of fast laquer thinner to thin the mix. This is actually part of the Duratec printed instructions.

And I’ve found the small Chinese gravity guns seem best at spraying out unthinned Duratec, but the small cups mean it takes forever to spray a mold.

But the real problem has been random appearance of what I’ve been calling alligatoring. At first I thought it was styrene migration from the molds. But I made some fender flare & an airdam molds using epoxy based materials, and when I used Duratec to spray into the molds and produce the first parts, I got some more alligatoring.

This experience has led me to think maybe I’ve been spraying the Duratec on too thin, and I’m getting a lifting from the mold surface because of that. Main reason I’ve allways tried the Duratec is threefold. I have a gelcoat cupgun, and I’ve used it successfully to build door panel per molds, using orange tooling gelcoat. Worked fine, but awkward, not too accurate if the mold is complicated, and messy as the dickens.

With a spraygun, I can get suitable controll of what I’m spraying. I’ve allways applied a mist coat, and what I’ve thought was two wet coats of Duratec. Maybe I’ve just not put the Duratec on thick enough?

So if I switch to using a gelcoat to spray into a mold, just what one do I use. I’m producing car body parts that are assembled by builders, so a sandable gelcoat is desirable. Color doesn’t seem to be an issue, but grey is nice.

If you’ve bothered to read this far, I’d appreciate your input.

David Breeze

#2

Which Duratec are you using? The clear or what?

Duratec is a gelcoat type of PER and if you are alligatoring, it is most likey the gel coat is too thin or the lamination coats are too hot (too much cataylst) or too many layers at one time which shrenks and pulls on the gel coat.

It sounds like grey surface is ok for the gel coat so buy grey sanding Gel coat PER.

We also do 1 piece front ends and the molds are fairly large. We use the standard dump cup gel coat guns for them (there are different size nozzles available too) and save the smaller guns for small molds or clear gel coating a Carbon Fiber piece.

Also, you might want to try using a wet mil thickness gauge until you know how much gel coat to apply.

JM

#3

I use duratec in-mold as well, and I spray it with a 2.0mm HVLP. The thickness you have to spray and the method of spraying is counterintuitive.

First off forget mist coats. I had the same alligation problems and it’s because I treated Duratec like it was paint (hard to not do when you are a bodyman by trade ;)).

Anyways you have to treat duratec like thinned out, sprayable bondo (which it is chemically more akin to). Flow on a heavy coat. Make sure the fluid needle screw is all the way out, and keep the fan control fairly narrow, narrower than you would spray paint with- probably a 6" fan at an 8" distance away from the part. Flow it on in long, continuous stripes, do not sweep back and forth. You should be flowing enough on that it gets wet and shiny and ready to start making runs. You will definietly use a lot of material, but duratec assumes you are using the product because of its build properties.

Also, I’ve never had great luck doing layups with the duratec stick slightly tacky. It seems far less prone to lift if you give it a good two hours to cure.
It will not form as good of a bond as unwaxed gelcoat might, but it certainly isn’t a waxed product that will resist adhesion either, and I have not heard of anyone having problems with duratec delaminating from parts that were laid up on cured duratec.

Sorry of you knew all of this, but I had similar problems so thought I’d relay what I found. I’ll probably try gelcoat at some point, but duratec is really nice to work with once you get it down. And yeah when spraying multiple cupfulls be certain not to overcatalyze.

For something like a front bumper skin that might need 2 or 3 full cups, I tend to catalyze each cup a little more. Something like 1%, 1.5%, 2.0%. I suppose that might be bad practice but it tends to actually mean that by the time one coat is sprayed and the solvents flash, it’s sortof “on the same page” as the next, more catalyed coat. And the residue in you gun won’t go rock hard while you re-fill the cup. :smiley:

#4

I’ve allways used the 007-002 sanding grey Duratec. It seems to be the one they recomend for this type use.

I’ve tried starting the layup once thumbprint stage is achieved, tried waiting a couple of hours after thumbprint, and tried waiting untill the next day. Seems to make no difference.

I allways brush on a thin coat of resin into the mold when starting the layup, and apply a 1 oz mat skin, which I let cure overnight. Then follow up with ether a chopgun or if a hand layup I’ll apply one or two layers of 2 oz mat, wait overnight and layup a third 2 oz mat.

Chopgun is set at 1 %, and best I can tell, I’ve hand mixed at 1% or slightly less, depending on temperatures.

The absolute randomness makes me think thinness is the issue.

When I shop (online) for a “sanding gelcoat” I just don’t find them. Are all gelcoats considered “sanding”.

David Breeze

#5

Captian Bondo…I really like that screen name :>

Your right on, I’ve been trying to spray it like it’s paint, so I’ll give your methods a shot.

And I too have found that the recomended 2% catylization of Duratec seems to make it gell right-fast. Faster than I can spray out a litre anyhow. Droping down to 1.5% seems more my speed.

If I can solve my problems, then Duratec is just fine. I like it’s qualities, once a part is produced with it. My parts allways come out with that nice gun metal gray shiney surface, and customers really like it.

David Breeze

#6

LOL, yup. Spraying a full liter at 2%, especially when spraying it “like paint” , meaning it takes a bit of time, generally results in you getting to tear your spray gun right down and spend an hour picking the duratec crust out of it. I think it’s a very common first mistake, it’s certainly what I did.

Definitely hold a rag over the nozzle when you are done to “back-blow” the material in the gun passages back into the cup. Then wipe the cup, put a few ounces of acetone in the cup, spray it out, back blow again, and give it one more wipe.

#7

just a question but how long are you keeping the parts in the mold?

#8

Problems,
I usually let a part set a minimum of three full days before pulling from the mold. However, I have done full layups and left them in the mold for 6 months. My current molds were actually in storage for 8 years, with a full layup done in them. The parts all came out nicely.

I’ve never had a part sitick in the mold. I initially prepared the molds after removing from storage by polishing with Fiberglast step 2 polish and MeGuires machine polish. Followed this with 5 coats #2 Partall wax and two coats of PVA. Every layup has parted wonderfully, and I’ll either add one more coat of wax or else just wash them well and PVA for the next layup.

#9

You mentioned that it takes too long to cover the mold. Seems your compressor can’t keep up, what size compressor are you using?

#10

cfcrafting,
My compressor is rated at 19.5 cfm @ 100psi free air, produced by a 40 amp 5 hp motor and a 25 cfm pump, and stored in an 80 gal tank.

http://www.eatoncompressor.com/catalog/item/504747/172993.htm

I got it based on the Venus/Magnum system requiring 17 cfm @ 90 psi.

The compressor is able to keep up, I understand your thought. What I was saying is that on the two larger molds I have, I’ve been spraying on about 1/2 gallon of Duratec. That means I’m filling up a typical paint-spray gun three, four, or five times. It’s not like shooting a small mold with one batch.

I prefer the “precision” and control of spraying Duratec with a paint-type spraygun. I’d switch to Duratec and a cupgun in a heartbeat, if that was the answer.

I wonder what would happen is I hand brushed one of these big molds with Duratec?

And I’d try using a “sanding” gelcoat, but for the life of me, when I look online at gelcoat listings, I don’t see the word “sanding”. What is the deal?

David Breeze

#11

“I wonder what would happen is I hand brushed one of these big molds with Duratec?”

I have been tempted to try this too- it’d be horrible trying to finish a plug that way, but when the brushed “finish” is on the “inside” of the part, it might work good. I’ve heard you can get problems if you don’t keep the application of gelcoat very even, but I suspect duratec is more forgiving. I have a mold I’ll try it on, but that’s a couple months away. Let us know how it turns outif you decide to try it.

#12

I’d wouldn’t want to brush or roller the gel coat on a part as there will be lots of uneveness and after applying the reinforcements, you will have lots of air voids or pockets to easily break open.

Since you have the air compressor, buy a gel coat cup dump gun as they come with many different size nozzles. It will be fast and leave a nice surface to place reinforcements down correctly. You can get disposable cups from Smart & Final cheaply, or with the stock PE cup, you can get disposable PE bags for the gel coat to be place in the cup for a fast clean up. Time is money…

#13

That’ll do the job:-)

How about one of these http://www.lemmer.com/225-pressure-pot.htm

#14

That’s what they were using when I worked at a car audio manufacturer to shoot the gelcoat and PVA. Worked great as per my conversation with the engineer that was using it.

I personally just made the investment in a pressure pot myself along with a compressor, vacuum pump, another smaller sprayer as well as a ton of mold making materials…I’ll post back how well that pressure pot works. :slight_smile:

Dan

#15

Hey Dan, how’d the pressure pot work out for the duratec?

#16

you do know this thread is 8 years old.

(I did find it very interesting)

#17

It’s an epic bump, but looks to be some good info, no harm, no foul on the almost a decade bump!

#18

It is a good one.
I have been using Duratec 1799-006 Vinyl Ester primer on almost all my bodywork. I was told by the rep at SAMPE that it would work better with epoxy resins (I use PTM&W)
I shoot it out of a 2qt. pressure pot with an old DeVilbiss JGA-502 gun (not thinned) with very good results.

#19

Who cares if it’s still relevant to me? Would you prefer I ask questions that have already been asked but not completed simply to have a monitor tell me to search? Jeesh, let’s learn.

#20

to tell you the truth , and not to be rude i think it is the size of of your tip that is giving you the problem considering whenever i spray gel i never use a tip smaller then a 2.5 with most of the time being a 3.0 ( which is recommended ) depending on the job or size of the mold with a few exception’s ( big molds ) i will use a 7.5 tip . http://www.spraygunworld.com/Information2/GelCoat/GelCoatStart.html If you get a chance take a l@@k at this link thay offer good prices , and info to boot .