Special materials test: Between Body filler putty, Plastic Wood putty and Rock hard Water putty;
1. The overall strongest road/ special building material seems to be auto body filler putty. This bond well with all other board surfaces and is fairly dry so won't flow from where it is set. It dries fast on the surface but takes awhile to harden at depth.
2. Plastic Wood putty is good to and can be spread very thing and still take groves that look like tire tracks. This dries fast when that thin so only a small area should be worked. This if moderately strong but is flexible so pressure would bend it and not break it.
3. Water putty is very strong to a point. After this point is reached it will crack and break. It does bond well with board surfaces. It is very slow drying and will probably flow if used to make rock faces on a gully. The advantage of drying slow is a large area could be worked (say a stream bed) at one time. It took almost a week for one ¼" mound to dry thoroughly.
Conclusion: The cheapest material is the auto body filler putty (when bought at a big auto supply store in bulk). Water putty is next cheapest and is pretty good for most things. It may crack if used on a surface that would be exposed to pressure when transporting or stacking the boards. So don't use it on hill tops. Probably very good for shell craters or stream beds. It is slow drying, taking up to 3 days to thoroughly dry to the touch. Since it takes so long to dry it can be worked at a easy pace.
Wood putty seems to be able to flex under pressure so would be useful in places water putty shouldn't be used. It makes nice road material as it can be spread real thin. Body Filler putty can just about go any where the other two are used. The only draw backs are it smell and the price.