Bonding PU foam to GRP

For an experimental hull modification I need to close and fill 2 cavities with PU foam so no water can get in.
Would it be a good idea to apply Sikaflex sealant to the polyester surfaces and fill the cavity while the sealant is still tacky, or will the foam adhere just as well to a dry, degreased surface? And how much foam should be used to obtain a closed cell structure?

CDK

 

The two part foams I'm familiar with will stick really well to cured polyester resin. You want to figure out how much volume you have in the cavities, then carefully measure the foam you buy. Don't completely seal the cavity after you pour the foam in. The excess will spill out, kinda like beer head, pouring a 12 oz beer into a 13 oz glass.

 

The only pu-foam available here is the single component type in spray cans, used for isolation and mounting doors and windows. I've used it in my boat in dry areas but I'm not sure how it behaves when submerged.

 

The spray pak foam is not great - other forum discussions indicate that it will soak up water.
You might be able to cheat a bit - paint the cavities with a couple of coats of epoxy resin, then apply the cheap foam. The epoxy will stop a big percentage of moisture that polyester wont. Lastly, cover the opening with epoxy, not polyester.
Expanding foam can deform the sides of hulls, even when the opening is left open. The trick is to apply about a cubic foot at a time, then leave it for an hour bwteen each application.

 

CDK- have you tried ordering some 2 part foam online? I get my supplies from www.uscomposites.com- and no, I don't work for them LOL, but they do have great customer service!

 

Mongo75, they do not ship to Europe. It used to be possible to send a can of paint to the other end of the world by mail, but by now there are too many laws and regulations, both in the US and here.
I'll find a way though, probably travel by car to Italy and buy it from a marine shop.
How do you apply 2 part foam in a cavity? Is there a tool like a hand pump?

 

Usually you just put the two components in a container in the ratio stated (usually 1 to .5) , and stir like crazy. The trick is to stir enough that a good mix gets the most expansion possible, but if you stir too long it gets so 'fluffy' (like merangue) that it wont pour into a void.

Do a few small test runs first, and you will soon get the hang of it. It also varies with the temperature and humidity you do it in. Its actually quite fun!

 

That sucks that they can't ship it anywhere- how the hell is one to be able to do any work that way??? To fill a cavity, you mix, then pour, most would say to pour in layers, let's say yu have a void of 1x3 feet. Mix a quart, pour it in, and wait about 10-15 minutes for it to fully expand. That will give you a general idea of how much you really need to fill the void, without pouring too much and wasting it. If you haveto fill a void that's already covered, then drill a decent sized hole, and pour it in with a funnel. Be especially careful doing that because foam will easily destroy a boat, similar to putting a small explosive in the void- no where for the expansion to go, so it'll tear apart panels and such.

 

Just returned from a shopping trip in Germany, where I purchased spray cans with a special PU-foam formulated for sealing wells and sewers. According to the label it adheres to metals, ceramics and most plastic used for these applications. Closed cells, doesn't rot but isn't UV resistant, which is no problem for my submerged project.
Two components in a single can; after use I will cut one open to see how they did that.