In the interest of DRY BERTHS and other good things...
I'd first peel/scrape off the silicone
sealant. Often it just peels from
lexan and plexi. Then, because the silicone often leaves residual oils, use an appropriate solvent cleaner, i.e. 3m "Prepsolve" used to remove silicons and waxes before
paint jobs. The solvent needs to be something that won't attack the plastic but will attack residuals.
After the solvent, follow the instructions for the
adhesive.
Sanding alone sometimes just embeds residual solvent--so you prep clean before any
sanding. Then prep clean again to get rid of any sanding dust. The
adhesive may require that a special primer be used, and they're usually not
cheap but if you skip them, the adhesive may not
work properly.
Once you find out what the proper adhesives are, whether it is what GE (makers of Lexan) suggest, or what
Goiot suggest, whoever, the rest will be detailed in the instructions for that product. Anything from the top names should do the job and do it well, if they are followed, and if the hatches were originally built properly.
As to why they leaked in the first place...We'll blame that on the usual suspects. You didn't hold a proper wedding, didn't place a gold coin under the
mast step, didn't make adequate libations, so Poseidon is reminding you.
:-)