last week I posted about using caulk on foamboard maybe being a Catch-22 and the problems I had with finding a non-shrinking caulk to smooth over foamboard margins.
I just wanted to write this follow up so you all won’t think I’m crazy and maybe it will even be of some benefit to some reader some time somewhere. All this is used indoors in normal humidity and temperature.
Results:
Dap Silicone II products do not shrink. However, they also do not stick at all to foamboard and besides can’t be painted. What was I thinking? Luckily it all pulled up so easily.
Basic Dap Latex Acrylic caulk (with a little bit of silicone they say) in white also does not stick to foamboard. It can be rolled off the surface with a bit of thumb pressure. It also shrinks so is not so good for filling gaps or margins.
Dap Dynaflex 230 clear remains flexible, adheres very well to foam board (and d*%#@ near everything else) but can take 7 to 14 days to become clear and remains very tacky even after a week of drying. If one sets a tool (like a level) on a film of Dynaflex 230 for a few moments, the tool will pull up chunks of foamboard as it lifts up the caulk. The instructions say paintable after 2 to 4 hours, but it is very tacky for days and days
Basic Dap Latex Arcrylic caulk clear is a different product and seems the one to use. (It’s what I always used for holding cork roadbed to plywood and track to cork in some places). It adheres well to foamboard. It dries clear in a day or so depending on thickness, and its tackiness diminishes in the same period of time. Although it remains tacky over time, it is nothing like the Dynaflex. It may shrink over time I think, but hopefully not as much as the white one.
I guess my point is that even the basic Dap latex acrylic caulk is a much
I used a Canadian cheapo acrylic latex caulk, white stuff, for gluing all the extruded foam layers on my first layout, and it dried quite firmly and stuck that way, too. No shrinkage that I could detect, and no buckling in the foam that ought to have accompanied the process.
I will be careful not to use the DAP equivalent…thanks for the alert.
Since your description isn’t exactly clear, I’ll supply my answer based on the assumption that you are referring to extruded foam insulation sold in 2x8 or 4x8 sheets in various thicknesses.
If you are using these as a substrate for your scenery, I don’t think you need to worry about sealing a joint between two sheets with caulk, providing the joint isn’t as jagged and as wide as a canyon.
The foam, if cut correctly, should meet up in a tight, smooth joint.
Nothing that we model would be as smooth or as flat as a sheet of foam. Where I had joints, I used full strength white glue; spread a bead along the joint, smoothed it just a little bit and then covered the glue with sand. Once I had applied a base color to the foam, all of the foam was covered with a layer of sand and glued in place with thinned glue. The end result is small undulations in the ground, much like Mother Nature has done outside in the real world. In my opinion, that looks much better than a flat world.
When it came to gluing my layers of foam, I found that Gorilla Glue works great. Since it expands when curing, a little goes a long way. Just follow the instructions: 1) wet the surface of one of the items to be glued with water. 2) Apply Gorilla Glue (sparingly) to the other surface. 3) Clamp or place weights on items being glued. It’s dry in an hour, compared to 3 or 4, if not longer with caulk.
Sorry i didn’t repeat that what I am doing with this stuff is smoothing the margins of the 2x8 sheets of foamboard where they mee [and are out of level by 1/32 to 1/8 inch in places]. I was not speaking of gluing the foamboard to the plywood base which I had done earlier.
The reason I am being so fussy is that I have two tables where there will be wide yards with up to 7 tracks crossing these margins. On a couple of other pieces of benchwork I have just the single main line running across the boards. I haven’t bothered wirh caulk on these margins at all yet because ir will be a simple process of smoothing/leveling out just a couple of spots where the main line crosses.
Actually, I think you’re being needlessly fussy. You can just take a rasp and smooth the joints down, In those areas where you want it absolutely flat, use a cork sheet to “shim up’ the lower side (although I don’t think a 1/32 out of true is going to cause any issues, 1/8” might.