Hi Paul,
There are various ways of doing groundwork,everything i know i learnt from the members here on this site
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Firstly,will you be putting another layer of filler over the dio?
If so,for your next layer of spackle,add a fair bit of sand to the mix.After mix is applied to dio(while still wet)i tend to sieve some sand lightly over the dio(pushing into place lightly with my fingers),and place a little cat litter here and there to simulate stones.
I leave this to dry out for a day or two,then spraying a mixture of watered down white glue over the whole surface,yet again leaving to dry out.
The next step is to blow off any excess sand on the dio.
Now the dio is ready for a coating of brown acrylic craft paint,applied with an old paint brush.
Leaving this to dry out i then paint the dio over with an acrylic earth colour.Yet again,leaving to dry.
I tend to pick out the cat litter now by painting the pieces in a grey colour.
This is now a good moment to add a wash of raw umber and burnt sienna,you will see your groundwork coming to life now.
You can dry brush areas with various colours like Tamiya's Buff,it's all up to you!
This is by no means the way to go about things,the other guys will have other methods and tips to point you in the right direction!
BTW,the setup of the dio is very appealing and i like the concept that you are trying to bring to life.
Keep going digger!
Cheers,Shay