alright so here's the rundown.
firstly all my work is in paper, so not sure how well these techniques will go on foam.
there is no substitute for damage like real damage. so it all starts after the piece has been hardened. when using what is commonly called bondo on the outside of the hardened piece i add too much hardener so it cures fast and put it on fairly thick to it gives and uneven layer with pits and holes in it. once it dry i select out the bits i want to keep and then fill and sand the bits i don't want
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once you have painted it in what ever colour you want it's time for the rust.
i use a product called dulux design rust. it is essentially an acrylic medium with iron powder in it, and a rusting agent. which i get from the paint section the local hardware store
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you apply the iron paint on the areas you want and let it dry, now using a sponge you dab on the rusting agent, you dont need to use much at all, you dont want it running or any thing sort of like wiping down a table with a sponge you have squeezed the water out of. now you let it soak in the coat but not dry and flick spot of water on it. i use a spray bottle. the water will make it rust faster and create the lighter colour orage spots you see as opposed to the darker rust. some times you have to give it a few tries or recoats. where i live is quite hot and humid so it rusts faster. once you achieve a look you want you need to seal it with an acrylic sealer. i use liqitex gloss gel.
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once sealed it will stop air getting in and further rusting, i also use this gel because it retains brush strokes, dabbing it on with the paint brush unevenly will create extra texture for the rust. initially when you put it on the whole thing will go dark but once it dries the light orange bits will show again.
next is the grime citadel agrax earthshade is my go to for dirt effects but i have a games worshop close to it is easy to get paint from there, i'm sure you could just water down normal acrylics. i put the stuff on fairly thick as it dries lighter and i make sure to get it in the pits and cracks of the damage liek the dirt would naturally build up.
the final step is to accent the pits and cracks. i use silver rub n buff
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you don't want to use much of this, a small amount goes a long way. i usually put a tiny amount on the tip of a brush give it a few strokes on scrap to get even coat on the brush and then very lightly brush over edges and the tops of the cracks and pits making sure not to get it inside where i have put the earth shade. doing small sections at a time. it dries fast so i work in small sections.
and that is how i achieve my battle damage