foam core glassing question

hi, I will be doing alot of glass work on a new project, and really want to keep the weight and cost inline.

the foam will be 3/4 in sheet divenacell foam with a matt & biax skin, polyester resin. the question is for a non profesional glasser such as my self, would it be best to pre-impregnate the foam core with resin to insure a good bond, thus reducing the chance of the resin running out of the matt and into the small porous voids of the foam core ???

I do have a fair amount of experience with glass over plywood - and have had good results with foam core using the above mentioned method. but the whole point of the foam core is to keep weight to a minimum.

this will be a pilot house / cap rail / decking - bulk heads and stringers are wood.

thanks, mark

 

If you don't feed some resin to the foam, the resin will take it from your layups, so you've got to do something.

After reading others' opinions here, I am 85% convinced that the way to go is a wet slurry, which will save weight, but will stick better than a dry slurry which may cause a dry bondline. The cost of incorporating the slurry will be offset by the resin savings so it really shouldn't really cost much more at all to do it this way, it'll just be a wee bit more time consuming.


Ben

 

I usually coat the core with a light layer of resin, then set the glass on and wet out. The glass will wet out more easily and the core will adhere better.

 

If you let the resin cure before applying the first layer of glass, you ought to get an even better bond. You are absolutely correct that pre-wetting the substrate allows for easier wetting. However, the moment you lay down the first layer of glass, wicking action will draw some of the resin up from the core into the glass. At that point, you have the potential for a dry bond, just as if you never pre-wetted at all. If you're careful though, all you're really doing is adding an extra few pounds of resin (depending on size of the boat of course) so that's not a bad approach at all so long as everything is very well wet-out.

The problem though, is what happens on surfaces that are slightly vertical. Yes, you can wet out the resin, but you aren't going to be able to fill all of those little crevices in the foam. The resin will wet out the glass, adhere to the peaks of the foam, and then start to drip down. You, seeing that the glass is perfectly wet out, will then stop adding resin. Meanwhile, all the tiny little bubbles (crevices) in the resin will remain unfilled and the bond will be weak, despite perfectly wetted-out fiberglass.

Of course, most people know better than to try to glass fiberglass with straight resin to a vertical surface of foam. However, this effect isn't just on vertical surfaces... it's on any surface which isn't perfectly level. Add in the fact that the foam will absorb a bit of resin and the problem worsens.

So the answer is to add a thickening agent.

By mixing in a thickening agent with the resin, you use less resin (which ought to pretty much offset the cost of the thickening agent- silica, microballoons, flox, etc are all pretty cheap). Also, you will have a smoother, lighter, more filled surface to which to bond your fiberglass. Other than adding one step and one more thing to cure in the process, there really isn't any downside to using a slurry like this. There is, however, a slight risk of the slurry creating a dry bond due to the lack of resin mixed in with the thickening agent. Therefore, to play it safe, a little extra wetness is preferred over a little dryer of a mix.

Besides all that though, it may be interesting to note that resin, alone, isn't really a lot of fun. It's runny, drippy, and sometimes stinky. However, slurries, thickened appropriately, are kind of fun. It's like super-sticky peanut butter (or ketchup, or whatever, depending on how wet/dry it is) that you have to spread all over. At first, it's frustrating because you are making a mess, but once you resign yourself to the fact that a mess has been made, you feel kinda like a kid playing in the mud. Wheeeeee!!!!

Hope that helps!


Ben

 

the one way you could go about installing you core and works well is use corebond its made for this, lightweight so not adding much weight, spread out lay on your core run a plamsander over the core it helps pull the bonding into the core, works well on the flat or vert.

 

I concur fully with Toot, and use the wet slurry method on the foam core and allow to set. Then in a second step I lay the glass on dry, and wet out using a foam roller, and a rubber squeegee to minimise the chance of resin pooling in any area. It gives a very uniform wet out that does not use excessive resin.

 

Pescaloco, you should look into Nidacore, I have had good success with it with zero failure rate.Im not a fan of corebond products because they add unwanted weight and thats defeating the benefits of using core materials. Or you could vacuum bag it. I have used a cheap vacuum bag setup using a wet vac.

 

guys, thanks for the replies.

I will do a little research on alternate core materials. my only experience has been with divenecell - just a little tricky to wet out.

also thanks on the slurry technique, that sounds like the way to go - once you have all the little cell crevices filled no chance of the resin wicking out of the cloth, this way a person could better controll the wet out process, not overloading with resin to compensate for what will be sucked up by the foam.

thanks, mark

 

core must be primed

Prior to bonding, core must be primed before hand with your catalysed resin. Without priming, delamination is greatly increased.

Lightly priming the core so its has a satin like finish will fill the open cells of the diab core. If these cells arent bonded, air will fill these cells. Therefore the bond strength will be significantly reduced.

Swapping the core with too much will also leaded to an adverse effect. Air bubbles will be trapped within the open cell. A fine moe hair roller is highly suited to core priming.

Ensure the core will well consolidated too. Either throught vac bagging, or hand hand. Either way, try to achieve a 75% or great resin witness on the backsides of the core.

What type of core are you priming. Plain sheet or GS?

 

priming

thanks, buckle

it will be sheet 3/4 in divinycell foam, with a matt and nytex skin.

this is a home project, I have not vacuum baged before.
how does one achieve a 75% ratio as you stated ?

mark

 

If you applying core by hand, I would only recommend using scrim back core (also known as GS or groved/scored core). GS core is similar to that of foam in plain sheet except the core has been cut in 1" x 1" squares (like a chess board). The squares at held together by a thin scrim back mesh on the back side.

Scrim back is designed for curved surfaces and also for hand consolidation.

When applying the primed core onto wet mat (minimum 2 x 600gsm csm) or onto a core bonding paste like crestomer 1196Pa, you should place the core scrim side up. The scrim holds all the core blocks together, allowing rollering of the back side. Rollering action will consolidate the core and thus removing the air. Upon consolidation you should notice either the resin or crestomer being drawn up between the core squares and will be visible from the backside (or scrim side).

Imagine you have a 1 sqm core section which has just be applied. A 75% witness means resin or crestomer is visible on at least 75% or 3/4 of the 1 sqm.

If you cannot achieve a 75% witness, you have to question weather your core is truely bonded. If you, can you live knowing the fact your hull could delaminate.

Most forms of plain sheet will have a series of tiny holes drilled through the core to aid air removal. If you apply plain sheet by hand, the core will lift from the surface leaving a massive air void inside. I would note recommend applying plain sheet by hand as you will only be fooling yourself that you have a bond. Only vacuum bagging a core will ensure you have a uniform and continious pressure until the core has been adhered to the surface.

If you accidently bought plain sheet, I would suggest you vac bag it or replace it with GS?

 

thanks, for the info buckle.

the good new is the hull will be layed up in a standard open mold by a professional boat builder and is solid glass - the rest of the project will de done by me in a combo of plywood and foam core.

marks

 

Have another one for all of you and apologies in advance as I am sure its been answered a million times already but I can't find a direct answer anywhere to my question! As an amateur I am thinking of replacing a cockpit floor of a little downeaster that I own and someone suggested using an 3/4" Airex core as an alternative to replacing it with marine-ply, which was what was there before. Is this advisable, considering I have heard stories of Airex softening due to exposure to sunlight and extreme heat? Secondly how many layers of fiberglass would be advisable to laminate (top and bottom) to get the original strength back in the floor and what type of glass should I use? And one last one, is there truely consensus that using the slurrying technique on the core itself before applying any layers of glass is indeed the best technique? Thanks.

 

Use 3/4 inch nidacore with 2 layers of 1810 glass on each side. Its as stiff as plywood.