What to use to seal gas tank compartment
 

I had to change my sending unit in my tank. What is the best thing to seal the hatch with? No seal when I pulled the hatch, but now I have water getting in the compartment. 78 V20

I used white silicone. Need to redo it once a year so you can inspect tank anyway, so make it cheap and easy to remove.

make sure its outdoor silicone like for gutters so it holds up in the sun

Absolutely silicon - clean all around first, then apply sufficient bead so it squeezes out all around when you press the cover down flat. Easy enough to wipe away the excess than not enough and have gaps where water gets in.

One comment.......if you worked on the sending unit, might want to fill the tank to confirm the sealing unit isn't leaking before you seal down the cover. The V21 fuel tank is plastic, the gasket and small screws prone to allow gaps, and the screws need to be carefully wrenched down tighter after screw driving down seemingly tight.

I did just replace the sending unit, and yes I got water in my tank afterwards.