Hi all, I have been placing 2" foamboard down on my benchwork for the last week and have noticed some slight differences in height at the joints, as well as slight (1/8" - 1/4") gaps between sheets due to my imperfect cutting. What is the best way to address these for smooth(er) operation?
For vertical differences will a sanding block of some sort be able to blend the two together without tearing up the top surface? The vertical difference is very slight: maybe 1/16".
For horizontal gaps, what is the best way to fill and level the gap between two sheets? I can of course fill the gap with PL 300 or DAP caulk of some kind, but is there something better I should use? At one time I thought Woodland Scenics made some sort of foam “putty” filler …like a spackle for foam…
Woodland Scenics still makes their foam putty. It’s very similar to compound but remains a bit fiexible when hardened. It contains foam granules as part of its filler. It’s a bit weird to sand but works well. For filling gaps in foam joints it’s perfect.
There are cheaper products including drywall mud that would work and sand a bit more easily.
For filling steps of 1/16" or more you can buy styrofoam type sheets from Woodlands in precise thicknesses, it’s a type of foam board similar to that used in architectural modelling. It can be faired in by sanding.
Before applying filler maybe determine why the step is there first. Is it still possible to shim up the lower edge by slipping cardboard or similar shims underneath the low side of the join? You don’t want the joint to move further after you level it.
Even two sheets of plywood can have a slightly different thicknesses when butted together.
I layer cardstock pieces to smooth the transition. Three , then two, then one etc as you work away from the joint. Three would mean a pretty big difference in thickness. Topside to smooth the roadbed, not lifting the foam from underneath. Roadbed would go on top.
If you’re concerend about the cardstock disintegrating when watering for ballast, then sheet styrene will work…and might be a better mate for foam.
Great stuff spray foam is good for filling gaps. It will expand more than you imagine possible. I trim it with a razor blade or a razor saw. You can fill big holes too and trim with a sureform.
I am building on individual modules with adjustable legs. I can dial out any horizontal differences.
Took the words right out of my mouth, Douglas. I was just about to write the same thing.
Owens Corning recommends DAP 230 or HVAC Foil tape to fill gaps on 2" foamboard, but I can’t help but wonder if simple masking tape wouldn’t do the job.
That said, 1/8" to 1/4" differences in height at the joints seem like something more than slight differences. Vertical differences of 1/16", on the other hand don’t seem so bad. But, honestly, any differences in height or any gaps easily become problematic.
As careful as I was in building my layout with 1/2" plywood surfaces, all gaps needed to be filled and smoothed with caulk before landscaping or ballasting.
Any height differences at joints are unacceptable. You don’t want any humps or valleys in the track work. I run into problems with my yards with height differences no matter how slight. I call them “hump yards” as I watch freight cars and passenger cars roll down the unintended inclines.
Blue painter’s tape works for me to close gaps so does duct tape. No one notices the tape when it gets covered by paint and scenery. Very minor elevation differences have not been an issue with my trains running.
filler would not be required as the track will alleviate the slight differences by itself …if you want to fill for scenic purposes, the drywall mud or expanding foam spray bomb work well …
Use a good level and pay some attention to keeping things flat. Very slight grades will seem to work fine, but after a while you’ll run longer trains and perhaps see some stalling on those minor grades.
Don’t be satisfied with “good enough for now.” When you’re a couple of years down the line, fixing problems like this will be more of a problem.
I cover horizontal gaps with thin strips of plaster cloth, which will eventually get Gypsolite or other scenery on top of it. I like Woodland Scenics foam roadbed. It will compress enough to smoothly go over a small vertical rise once track is down, and once ballested you won’t notice it.
There must be a small discrepancy in both the plywood and the foam thicknesses. The benchwork itself was all built with a level and varies within a range of only 1/8" vertically when traveling around the entire layout ~ 50’. I suppose I could have bought a laser to get it even closer.
The vertical differences are small and I might be able to slip something between the foam and the plywood like card stock or styrene.
The horizontal gaps are larger. I hadn’t considered just putting tape over them… do you think that will hold up long term? I would be afraid I would somehow cut thru the tape while doing something else and end up with a gap again. I do not intend to ever tear down this layout. It is being built-in permanently (although, if I can bribe my wife in future years to get more of the basement I plan to extend it).
No one mentioned any kind of sanding down of the actual foam surface on top to eliminate the vertical difference - so I will take that as a sign that won’t work.
I do have Dynaflex 230 on hand so I might try that for filling the horizontal gaps. I will feel more comfortable knowing the gap is gone and I can’t accidentally expose it again down the road.
I should add: I’m well-versed in using Great Stuff. Love it for many uses. Not sure how well it would work in this application, unless I use the formula that expands the least, for window and door installations.
It does tend to leave a “pitted” structure when you cut into it tho: its like sectioning a bee-hive and exposing all the “cells” inside. Not sure how well this would look when painted, but I suppose at that point I could fill in the small “cells” with foam putty or some other sandable material that would paint the same as the surrounding board.
I didn’t know there were different kinds. I used the little red plastic tube to fill in gaps between sections of foam. I did not get cells that could not be covered by ground foam. I was very stingy in the application of the GF.
I have also used it for big gaps, say between the foam and a curved fascia. You do get cells the size of gaps in home made bread.
I like the idea of blue painters tape. That is cheaper, less messy.
Yes the painters tape I would not have thought of. Again, only concern there is will it hold up year after year? I suppose painting it into place would increase its strength and lifespan.
On Great Stuff foam, there are blue cans for window and door, green cans called Pestblock, red cans for the typical gap sealant up to 1", orange cans labeled “Fireblock”, and black cans labeled “big gap” for gaps up to several inches. I’ve used all of them before. Great material, but definitely messy!
Yeah that’s tempting! I kinda get the feeling I would be a guinea pig for some of these techniques lol. I’m told there are TV ads for Flex Seal tape where a guy tapes shut a hole in a boat hull and makes it seaworthy again. Maybe I should look into that [(-D]
I just use a rasp to smooth out uneven foam surfaces where the track goes. If the track doesn’t go there I leave it uneven just like the real world.
I like duct tape for the cracks, just to keep the ground cover from falling through to the underworld. I used painters tape in the photo below and it is amazing how it just disappears once covered.
I think the idea to repeatedly check level when laying the foam, and carefully shimming the underside of a gap if the pieces were level across the top, are sound ideas. If the idea is to get the foam down fairly ‘rough’ first, and then ‘cut’ the subgrade into the foam to correct level and transition, you might want to consider some variant of a hot-wire tool, which will eliminate the many crumbs from using a Surform (or its baby sister for fine work the Dermabrade).
My understanding was that sanding was the wrong approach in foam entirely, the grains tearing rather than shearing the material, and little removed pieces of foam rapidly ‘clogging the grit’ and just sliding. A metal milling cutter might work but the pain to align it using usual tools might not be worth the ‘fun’.
I’d also mention that nothing keeps you from using sealing material and tape from making a trough in the foam and using self-leveling cement or similar material in places to get finish subgrade. (You can reinforce such a cast internally, and then tilt the result slightly after it sets if you want ‘plane’ subroadbed surface with a grade, and work out how to make the correct vertical curve to level sub…)
You want the vertical joints to be as smooth as possible for the trains. The roadbed will tend to sink into the lower section over time, unless you’re unlike most people and ballast right away and harden it up with glue. Some sort of support under the roadbed void would be wise.
I would think it would be difficult to sand the foam down to where it would get a gradual decent without the dip in the track and trains being obvious. Seems like it would take a lot of sanding to get the gradual grade, but maybe not.
I would think that if you had a vertical difference of 1/8 to 1/4 inch you’d almost have to have butted two different dimensions of sheet product by mistake.
