discussion Mixing materials on a diorama base, be careful!

[pjgtech]

Just a slight warning tale, but I'm sure I'm teaching granny to suck eggs....!

On my latest build, a small Desert Diorama with a ATMP x6 wheeler and Scorpion tank, I wanted a desert type or arid base type scenario.
I also wanted tracks left in the base to show the vehicles use, etc.

Its now finished, took final photos yesterday and was having another look at them today and I noticed some small cracks to the surface of the base, Bah!

I have had issues with warping in the past, just using foam board, so this time I used ply board as the main base, with foam board pva'd on top. All good, no warping, so a good result.

But I then used fine modellers filler to "rough up" the base and give it some texture. I then also used some Ammo/Mig mud paste to add more texture, small sand ripples, etc as texture.

I then carved out two small trenches in the foam board where the vehicle tracks would be, so they are slightly lower than the surround base surface.

Finally I then applied a very thin layer of Milliput into the two trenches, and before it had fully dried, I then ran the x6 wheeler spare wheel over the Milliput to give the vehicle tyre texture to the tracks.
NB: I used Milliput for the tyre texture rather than the two fillers mentioned before as it is drier and less "messy" than the other two, so easier to nice clear and clean tyre marks.
All good so far.....

Once it was all dry, I then primed, painted and added washes and pigments to the base as required.

I assume what has happend is that because I've used multiple materials to surface the base, they have dried, and shrunk, at different rates thus causing those small fine cracks?



Looking back now, I either didn't leave it long enough between adding the layers so the one below fully dried before adding the next one, or I should have just stuck to using one main material, eg: fine modellers filler, or Ammo/Mig mud paste or Milliput and not really mixed them?

Theres no warping in the base, so its not that, sticking the foam board to the ply base seems to have solved that issue. It just appears to be the join between the Milliput and the fine filler/mud paste that has slightly separated.

As you can see from the photos its not a huge issue, and now that I have noticed them, they could easily be hidden with more paint, more filler, more fine sand, etc. Lesson learned for next time....

 

[Andy T]

Often called mud drying because of how it looks.

Basically the surface has dried before the underlying layers. This makes the surface inflexible, and as the substrate dries, shrinks, and generally moves around it can cause that cracking.


It's a phenomenon that can be used to good effect, as in the crackle products you can get where a fast drying layer is applied over a much slower one, but a pain if it's unwanted.

 

[pjgtech]

Yes, I should have realised this myself, as I was a Building Surveyor for 30 years, so know enough about expansion and contraction and varying drying rates and cracking in materials.
When looking at cracks in buildings there is actually a "Classification of Cracks" table that can be referenced, Lol...

 

[Jim R]

Despite what it says on the tin true drying times are often far longer. Paints, glue, fillers, putties etc can be surface dry in the time stated but to fully cure can take much, much longer. I have found that even a simple decal setting solution can effect the paint if not enough time is given for the paint to dry. Water based acrylic paint can be touch dry in half an hour but may take 48 hours to cure completely.
I'm sure your excellent dio still looks fine. A few cracks will not be noticeable and may even pass for deliberate weathering

 

[scottie3158]

I'm sure it will not detract from the great finish.