mold making problem

[7521ply]

Let me start off by saying this site and the people on it are amazing! You all do some of the best restorations on boats that I have seen. I am restoring a 21' glasply, its a local boat from the pacific nw, sorry not a csc. I hope you all dont mind me posting on here, but from what I see this is the best place to go for the answers I need. I have replaced my transom with 1.5" coosa, built fiberglass foam filled stringers and am now working the back deck. I built a mold with mdf sprayed with duratec sanding primer, waxed it several times then another few coats of duratec followed up by a layer of chop and 1708 using poly resin. my problem was the duratec dried fast and as I was glassing it, it started to bubble the primer and lift it from the mold. I could not roll it out it would pop right back up. What happened? When you all do your layups do you wait for the duratec to dry or lay it up wet? any input would be great, I have several other molds I want to make and if I cant get this right its not going to be worth the time building the molds. Here are a few pictures to help show what happened. Thanks Josh

I hope this pic works!



If you all want more pics of the mold or boat let me know thanks again

[strick]

You have alligators. I read your post three times and I'm still a little confused as to what you did. You said you were glassing it... do you mean you were laying up the part from the mdf mold? or are you trying to make a fiberglass mold out of the mdf plug? At any rate you need to let the duratec cure completely over night before you wax and lay up the part. Try using PVA over the waxed mold. The picture appears to be a fiberglass part or fiberglass mold of sorts. Using terms like plug, mold, one off mold, part, fiberglass mold and being a little more specific will help us so we can help you better.

strick

[Blue_Heron]

Ditto to what strick said. Especially the recommendation to use PVA. If you can provide more detail on your process, it will be easier to help out.

From your post, it sounds like you first used Duratech on the mdf to provide a smooth surface, then you waxed over that and applied (sprayed? rolled?) Duratech on the mold to make the finished surface of your part. Is that about right?

I was wrestling with alligators on all my lay-ups until I figured out I wasn't applying my gelcoat thick enough. I haven't used Duratech's sanding primer, but it's polyester based, so I suspect it behaves similar to gelcoat, at least as far as alligators are concerned. I looked up Duratech's application instructions and they recommend 10-40 mils coverage. I would recommend you apply at least 15 mils, and wait until it has gelled before starting your layup. Strick waits overnight, I wait about two hours, or until the surface is tack free. 15 mils is about 1.44 oz./sf. So for a mold 5'x5', you would need 5 x 5 x 1.44 = 36 oz. of Duratech.

For the part you've made, all you can do is grind and fair the alligators and then paint or gelcoat over them.

All that being said, it looks like you did a great job with a complicated mold. By all means, post more pics of your process. Oh, and welcome to ClassicSeacraft.
Dave

[7521ply]

Thank you for the responses, I’ll try to explain a little better. I made the wood plug to make a one off part for the boat. The part in the picture is the finished product. I started with 1/2" mdf and some fir lumber to make it all. I then sprayed the wood with the duratech, the next day wet sanded it smooth, and waxed it 4 times throughout the week. It then sat for a few weeks before I could work on it again. Once I got back to it I sprayed it with the duratech again waited for it to gel up which happened very quickly and started to do the layup. It was about 20mins into the layup I started to see bubbles coming up where I had already finished. At that point it was too late to do anything to fix it. I think you guys answered it for me, I need to let it cure more and also apply a thicker coat of duratech. I am going to take your advice on the pva instead of duratech. Thank you for the help, and here are a few pics of the mold in various stages

here it is bare wood



here it is just before layup



and here it is layed up



Thanks again Josh

[Blue_Heron]

I am going to take your advice on the pva instead of duratech.

Don't do that, Josh. Use the PVA after you wax your mold, but before you apply the Duratech that makes the surface of your part. PVA is a water based mold release that forms a plastic film between the mold and the part. You have to spray it on, preferably at about 60 psi so it mists.

I see in your second pic another possible source of alligators. Looks like you used modeling clay to fillet your inside corners. I learned that from strick, and it works like a charm. But, the oil in the clay can mix with the resin in the Duratech and cause it to alligator, so again, the PVA is important. If you spray the mold with PVA after you complete your clay fillets, it will seal in the oil and protect against alligators.
Dave

[floorboy]

I think your finished part looks pretty good. You couldn't just fair the bad spots and be done with it??

[7521ply]

That helps a bunch thank you Dave! Thanks Floorboy, my plan is to do what I can to fix and smooth out this part. I just hope in time I can be as good as other members who make molds and to not have to do more work then I need.