>>Guess it all depends on what kind of plywood I end up using.
Your last sentence says it all. It really depends on what type of plywood you use. I would not stain AC or AB pine plywood. Both tend to have a lot of repairs done to the 'A' side that basically look like small footballs. They really stand out when stained.
>>What is better for the inside of a cage, staining it or painting it?
What is better is what you prefer. I've seen some cages with painted interiors and stained exteriors that looked fantastic. I've also seen some that looked pretty bad. I don't think I've ever seen a cage with a stained interior and a painted exterior, but I guess even that would work.
A recent poster built a Uromastyx cage that was painted on the outside but only clearcoated on the inside. That also looked very nice, not that I saw it in person.
>>And if I paint it, do I have to use a layer of clearcoat too?
No, there are paints that can stand by themselves. May need touching up, but they don't need a clearcoat.
In a nutshell, decide what type of plywood you want to use and we'll help from there. If you are thinking about a combination of paint and stain look no further than birch-laminated plywood. Sometimes called maple or mahagany plywood, but anymore they're so similar it does not matter.
I agree with Toon - don't paint Oak unless you really like the look. Some people do like the small amount of grain showing through or even doing a wipe so only the grain is filled. It's just not for me.
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Current snakes:
1.1 Gonyosoma oxycephala - (Silver/Yellow)
3.4 Gonyosoma oxycephala - (Green)
2.1 Gonyosoma janseni - (Black)