Hull Skin

[TSITL]

At the moment, I'm looking at making my fiberglass mold right on to my frame. To do this I'm currently looking at wrapping the outside of the frame with plastic, and backing it with spray-on foam, an inch thick to be the same depth as the frame depth. Then making the outside surface smooth with a kind of plaster of paris. Then putting the fiberglass on the outside and inside.

I was wondering if anyone could tell me if this is just a bad idea to do, or what significant drawbacks this idea may have. Thanks

[Dutch Peter]

Originally Posted by TSITL

At the moment, I'm looking at making my fiberglass mold right on to my frame. To do this I'm currently looking at wrapping the outside of the frame with plastic, and backing it with spray-on foam, an inch thick to be the same depth as the frame depth. Then making the outside surface smooth with a kind of plaster of paris. Then putting the fiberglass on the outside and inside.

I was wondering if anyone could tell me if this is just a bad idea to do, or what significant drawbacks this idea may have. Thanks

I think the problem begins with getting the pray-on foam in a layer precisely 1 inch thick.

Is the frame just a building frame or will it become part of the boat?
If it's a building frame, cover it with thin sheets of ply (diagonally) this will give you a good base for the lay-up. You only need to "plaster" or filler the seams.

With some more info, I can help you further.

I tried using plaster of paris once on a plug. It didn't work worth a $@#*. Its very slow to dry. It shrinks and cracks. It doesn't adhear well so it breaks loose when you try to sand it. I ended up scrapping it all off and using bondo.

[Dutch Peter]

Originally Posted by des

I tried using plaster of paris once on a plug. It didn't work worth a $@#*. Its very slow to dry. It shrinks and cracks. It doesn't adhear well so it breaks loose when you try to sand it. I ended up scrapping it all off and using bondo.

Sorry guys,

What is??:

- Plaster of Paris??
(is it the plaster you use in houses?! - DO NOT USE THAT ON BOATS)

- Bondo??

My dictionary only goes so far.

Yes , plaster of paris is similar to the plaster used in houses. It definately should not be used in boats.
I may have misinterpreted the question. I assume that the boat frame was covered in foam and filler to make a plug. The plug is then painted and sprayed with mold release then covered in fiberglass to make the mold.
Spray on foam and plaster will not make an adequit core for a boat.

Bondo is a cheep polyester base filler commonly used in auto body repair. It should not be used in the core of a boat either but it is adequit for a plug.

[Dutch Peter]

Des,

Thanks for the explanation. I don't think you misinterpreted, I also assumed the foam and plaster are to be used as mould material and not as core. My remark in capitals put you off, I think. Paris Plaster (plaster of Paris or Plasteris???) is also not suitable for moulds, that's what I was trying to make clear.


TSITL,

Do not use above materials as core!!!


Regards,

Peter

[Tall Timber]

Dry wall 'mud' might work for a one off mold. Doesn't shrink and crack a huge amount if applied kinda thin. Cheap, easy to use, washes off with water. The mold would probably not survive for reuse though. Just make sure you wash it all off before you launch the boat so ya don't get a ticket for water pollution.

[nassaw]

I think I understand what your trying to accomplish and you may be on a right track. Closing in the back and spraying foam to fill the voids between the frames. Fill just a bit higher than the frames and board back down flush. Carefull whatever you use to skim and fair. Anything that cures harder than the foam such as bondo will sand differently. The foam will sand away leaving hard spots higher. ATC Chemical makes several fairing products. I use Polyfair every day which is a great product but not to be used on foam. I believe its Poly Bond they advise to use on the cove and bead foam system. Sands about the same as the foam. If the low spots are light. Go with a fairing primer. Duratech blue or poly fill. Can be sprayed wet on wet to a thick build and sands easily. Hope this helps