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Can you take a picture? You should not have any gaps to see into the inside of the model. So you have something going south there.
Also, PLA is not waterproof in general and doesn't play well with most any chemicals. Do it right and use PETG, which is waterproof. It's actually pretty impervious to most chemicals, including hydrocarbons.
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Huh? I've done a lot of chemical tests with PLA and found not much harms it besides removing the pigment. Acetone, alcohol, toluene, Tetrachloroethylene are the main ones that things I print in PLA come in contact with. Long term submersion in toluene and tetrachloroethylene make it kinda soft for a little while but it has to be total submersion for multiple days at a time. Splashes and cleaning have no real negative affect.
For the OP, good luck with water and air tight. I worked on designing a 3D printable diaphragm vacuum pump for a while. I found that I could not make it efficient with FSM printing and had to go to resin printing to do it. With a resin printed pump I can see as high as 15inhg of vacuum and I have one that has been used on a production line for almost a year now with zero issues. It gets exposed to toluene fumes.
The same pump with FDM printing is maybe 5inhg of vacuum.
You will have to over extrude to try to seal the inherent gaps created by FDM printing. Print hot, slow and over extrude a little to have the best chance. It can be done but isn't easy. I had issues with every material I tried to FDM print, even stuff like nylon.