You are looking for photoresist. Comes in two kinds: liquid or spray-on.
Make sure you find out the process requirements BEFORE you buy, some of
them require rather special processing, are poisonous, smell bad, and in
general dangerous. One that is much used by amateurs is Photopositiv 20
from Contact Chemie in Europe (but available everywhere afaik - even
here). There are other options, like Kepro, 3M and many others.

Peter

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