What I have done is use toilet paper to fill the spaces. Take about 6 squares at a time in your left hand, wet your right hand under the faucet and scrunch the TP dampening it. You just want it manageable not soaking. Tear off what you might need for a wheel well test first, something without the fiddly bits like seat belts and gun sights. Tease the paper into the corners and little areas with a pin. You will find it doesn't have to be Forced into the cavity, it can be loose, you are just trying to protect the area you don't want painted, not trying to make a mold, if you get my drift? When it begins to dry it will take the shape of the cavity and will not fall out. It will be light enough to pull out with tweezers and not harm the inside. It works for me. You may have little points to touch up, but not huge areas. Bill