Epoxy over XPS method

Any recommendations about the steps for applying epoxy glass over XPS - pink foam from HD (XPS acting as a core and glass on both sides)?

• Some tutorials apply epoxy first and then the cloth over it. Here is the example starting at 2:27 and they actually recommend two layers of epoxy on XPS before putting glass down.
  
• Other tutorials put cloth first (over dry XPS) and then epoxy (here is the video from Totalboat or another video (starting at 2:10).

Is one approach better than another? I tried three approaches on test pieces with similar looking results but would like to use approach that provides most structural strength in long term:

1) thin layer (or two) of epoxy over XPS; let it get tacky (or let it dry and sand). Then a fresh layer of epoxy and glass on top of it (epoxy penetrates into glass from the bottom and wets it out.
2) Same as the first step but without any extra epoxy layers in advance - just a fresh layer followed by cloth on top immediately.
3) Cloth over dry XPS and then pour epoxy on top and wet it out. The third method is easiest since it is easier to lay out cloth on dry XPS without distorting it.

Is there a better method out of the three? Every resource gives different recommendations on the web and do not want to take the easiest route and then regret it couple years later.
thank you,
Mike

 

Method One/Two is my choice.

I like to majorly abrade the xps with 36 grit floorpaper and the only way to avoid air there is a thin hot coat.

Letting it dry is a mistake because you will get a weaker bond and have to deal with a little hump of thixo instead of just consolidating it.

Tacky surfaces can be tricky with light glass which is why you like #3.

But you can also wet roll over the hot coat and lay the glass down.

I make reference lines with a sharpie anytime I glass over wetted areas. This really helps...lotsa arrows and words making sure you repeat dry layouts. I load glass on 3-4" rollers always. This helps keep the lengths from going wild in stretchy materials. You can even pull up one time and adjust a few feet if needed.

 

XPS is basically non structural.
Your putting lipstick on a pig, IMHO.

The better method is to use a better foam.

 

Well, he doesn't even mention the application. I am using it for a livewell...certainly not a hull.

 

"would like to use approach that provides most structural strength in long term:"

I can agree for a livewell. Then it won't matter which method, cause the strength of the foam pulled from the epoxy is very low to start with.

 

Hugh Welbourn, inventor of DSS, built a 27' prototype out of EPS-and it is still racing today.
Take a look at this: https://www.achfoam.com/ACH/media/ACH/docs/Foam-Control/EPS-vs-XPS.pdf

 

There really ought to be a law against using epoxy resin on polystyrene foam. Isn't there one ?

 

No, No. One of the best things to use on polystyrene foam is epoxy.
Because it doesn't melt the foam.
Of course Titebond 2 doesn't either.

If you want to use EPS as a form and put enough glass/epoxy that it is not really needed that works just fine.
The foam itself is not structural enough to do a "good, lightweight" job of improving the stiffness.

Try this, make a foam panel. cut out a 2 to 6" wide strip with a block of wood in one end. 2 feet long.
Clamp the panel on the wood insert end so it is horizontal.
Draw a vertical line on the foam.
Load the panel with a weight so you get some curveature.
Look at the line in the foam.
A good structural foam will result in the line still being straight, because the foam is providing shear stiffness.
The EPS will result in a curved line. Because the foam is bending, since it is not stiff enough to contribute to shear stiffness, which improves the bending of the panel.
For EPS the stiffness is primarily the bending of the facesheets - very little "sandwich" type stiffening.

If you have a structural foam vs EPS with the same thickness and equal laminates, the structural foam will be significantly stiffer and will carry a much greater load.

Sorry, I couldn't help it. Back to you.

 

I built my dining table in the saloon of my cat from XPS sandwiched with thin plywood either side. The result was stiff as concrete and looked great
XPS can be a very useful material - it shouldnt be given such dismissal so easily as the forum members here like to do. Yes its a weak foam compared to your structural foam cores for hulls and things but you can find many uses for it in other places on boats besides a hull. I beleive it also becomes more useful when you laminate something like plywood either side of it as it protect the core from crushing and impact or compression type damage. You could even build a hardtop roof from it and it be plenty strong enough to jump all over it when its in a ply sandwich to protect it from dents. When you have a huge surface area - the peel strength of of epoxy and the foam is not really a limiting factor for many type of applications. Remember they make truck bodies from XPS sandwich panels and they last for decades bouncing down the highway flexing and moving etc- its pretty good stuff provided you dont try to load it in a way that exceeds its material properties - particularly in compression.
If you wish to laminate just glass directly to the foam - make sure its a rather thick layer of glass if you need it cop any sort of impact. Even for an icebox - just throwing in a few blocks of ice of frozen fish will damage a thin layer when the foam has little resistance to crushing underneath it.
I favour the laying of dry glass and pouring resin over the dry glass and wetting it out "through" the glass - if the foam is any colour other than white - you will be able to see if the resin has fully wetted the core and bonded to it properly. White is a litle harder to see whats going on and you have to be careful.
Its easy to position everything first, you get a neater job and make less mess in the process.
I do not sand the foam first - epoxy has enough bonding energy in its own right to destroy the relatively weak foam if you try to peel it off.
If you wish to lamainte ply wood - i wet out the ply first with neat resin and then mix some extra resin with cabosil to make a thick glue mix. Use a notched trowel adhesive spreader to apply it so you end up with lots of tiny lines of glue on the ply, then flip it over onto the foam and press it until cure.

 

Yesterday, I built a giant shim for a livewell base using xps. I ran the xps through a timesaver sander to try and make it uneven.

When done, I sanded both sides with 36 grit floor paper. This is messy, but modifies the surface greatly for adhesion. The HDT of xps is lower; it will melt from hotglue; another negative as it would be more usable in places where you might want to use the hg; like furniture or non BH walls.

Here is a fun pic of some rough sanded xps. Zoom in and you can see the modified surface I prefer. It is so messy to modify, but worth it, imo.

Based on experience and G remarks, I am putting 12mm corecell over the top of my shim; so if we ever drop ice into the well: it won't delam or crush as it may with xps. The sides of the well and the top will be xps; less subject to impacts... and I get great r value from a cheap 2" foam.

 

My method of abrading might be overkill, but I have 4' canoe outriggers in xps with 6 oz glass and they get rough handling and never delam ed.

For a hull; I'd have crush concerns.

 

Grouper,

I agree 100% about non boating, non structural uses of EPS.

And I agree there is no need to sand the surface - unless you think there is a contaminate on the surface.

 

And you're confused about the primary benefit of using a foam sandwich in the first place. Structural foam is an extremely misleading phrase. If cost is no object, then a stronger foam is clearly better than a weaker one. But in any and all applications of a foam sandwich the strength is coming from the composite and the space between the layers making for a stiffer structure as compared to a simple shell of the same composite. If your foam is providing a significant portion of your strength, then you might just suck at composites. There are exceptions to this, but by and large, XPS is fine for composite construction. Polystyrene has been used on homebuilt aircraft for going on 50 years, and if you think that marine applications are more prone to fatigue or need to be stronger than those in aviation, then you have some reading to do.

 

EPS usually refers to expanded polystyrene. OP is using extruded polystyrene or XPS. They have very different properties.

 

Dsigned,

I worked in aerospace structural design.
I've built EPS beams and tested them for deflection.
I spent a lot of time working on minimum weight designs.

You can use EPS or XPS and make something that will work at a cost in weight.
You can use either to provide stability for a thin laminate, but not at minimum weight.

I once suggested a coworker not fly an aircraft with a EPS structure due to deflection of the control surfaces under minimum load.
He didn't die in that aircraft.

Marc
Retired - finished with reading that stuff.