Here is an easy way to create that look of a vehicle haveing open sores on its armour (for a "living vehicle" or a vehicle of Nurgle)
I started by putting pools of white glue in the places I wanted there to be sores
Then while the glue is still wet, I put drops of super glue on top of the white glue
Give it about 24 hours to dry, and there will be there sort of wrinled sections in the middle of a crater
I took an exacto knife and cut out the circle in the middle (basically cut away all the wrinkley looking stuff) and then that leaves you with these nasty looking craters
You can then rough up the inside of the craters on the actual plastic of the vehicles if you want, or just paint them as is. They may look weird at first, but after you put a coat of paint over them, they stat to look more like these craterous sores. Cool look for a nurgle vehicle.
If you wanted to go the extra mile, you could then use "green stuff" or modeling apoxy and make tentacle that come out of the craters and creep onlong the vehicle.
Let me know how it turns out, you can practice on your extra tank bits or extra model bases if you want to get a feel for how it will look. If you have other ways of doing this, let me know, I am always open to knew ways of converting my models!