Prepping foam for skinning

[luthienon]

How much prep needs done to the foam surface before skinning?

I have read a few methods for what needs done. One was to use a wallpaper roughing tool on the surface. Another I had read was to use a sander to rough the surface.

I am inclined to approach it by using a sander on the surface and roughing it all up to the point that it is no longer shiny. I assume that would be an improvement for adhesion for PMF or filon… right?

 

[Pmullen503]

You definitely want to sand past the shiny, melted surface of the foam. I'm not convinced that the wallpaper roughing tool is needed.

 

[KCStudly]

Be careful and test on scrap first if you're going to try a power sander. Even a hard board by hand can cause gouges and digs if you aren't used to how soft the foam is. Power sanders can get you into trouble very fast. I even had issues with medium and long boards, especially when lifting off. The corners of the board can catch and dig in. When using very coarse grits, even with moderate pressure when you slow down at the end of the stroke the grit can bite in and tear crags in the foam, more like ripping at the surface than sanding. Test on a big enough sample to get the feel for it.

 

[luthienon]

Thanks for that advice. I will have plenty of scrap to test with an orbital sander. Maybe I need to break out the dual action automotive sander?

 

[Pmullen503]

When sanding foam, you should sand the same way you would pet a cat; in one direction only, lifting the sanding device off the surface for the return stroke. At least for finish sanding and leveling filler, a flat board or piece of plywood with sandpaper, or a cut sanding belt, glued to the board gives a very flat surface when used correctly.

On my first trailer, I used an orbital sander to remove the glazed surface. To this day when the light hits it at the right angle, you can see exactly the pattern I used.

 

[DrGofast]

I’m not doing a long book here, just adding a few small tips for Foamy DIYers:

1. When cutting or sanding XPS, note that its manufacturing process (under heat and pressure) — extrudes — the melted polystyrene and a blowing agent into the final linear board-stock. The resultant product then has a somewhat elongated grain (analogous to wood cells). Thus, you should note, it cuts and sands somewhat differently, ripping vs cross-cutting.
2. XPS can provide terrific material properties for building Foamies (fabrication, structure, thermal, moisture, cost, etc.). But, use caution re high temperatures. Manufacturers list about 160 degrees F. (71 degrees C) as max continuous temperature. Note that certain exposures to heating elements, exhausts, electrical devices, passive solar collectors, etc. may exceed 160 F. XPS may then bend, distort, shrink, or melt back from the heat. And too, it’s generally best to coat outer protection layers with direct sun exposure, in lighter/more reflective finishes

References: In past careers, I have been A&E technical resource for two of the three colors (blue and yellow) I.e. companies, making XPS in the US (no current connections).

 

[DJ Davis]

I took a large can top, 7"...8"...? diameter, used tin snips cut "teeth" in the edges, and folded it over into a half-circle. Holding it by the flat edge, I'd drag it in an overlapping "X" pattern until the surface was no longer "shiny," as mentioned above. Yes, that process caused "pilling," but also provided voids for the glue to flow into, like the holes of the wallpaper roller leaves with each pass. To remove the pill flecks, I hand-sanded with very coarse paper in a hand-held sanding block. That cleared the foam flecks and ensured the entire surface was roughed up. A few passes with the floor attachment of the shop vac got everything smooth and clean. I then proceeded with my Titebond-II coat and 10 gauge canvas PMF layer. She's a "garage queen" when not on the road/camping: seven (7) years operational with 18500+ miles of road time. I buff her out with those "magic erasers" once a year and she shines like freshly painted after that.

 

[ghcoe]

I just use a sanding block that I get from the Dollar Store. Just need to sand enough to get rid of the sheen.

.

Part of my panel prep for canvas now includes applying a coat of bonding primer. I find that I get a better canvas bonding with the it. The primer has a better bond to the foam and TBII bonds well with the primer.

 

[rruff]

One was to use a wallpaper roughing tool on the surface. Another I had read was to use a sander to rough the surface.

I used both for epoxy/fiberglass... the spiky roller. Work the epoxy into the indentations and wait for it to begin to solidify before doing the layup. It makes the weak foam stronger with better adhesion.