DIY gasket?

I was thinking similar. A bead of Silastic around the perimeter on one half of the box, coat the mating surface of the other half with a smear of vaseline and assemble.

If you could partially pull the two parts together until the Silastic cures, so that the bead ends up a little bit thick, that might be good. Then when you fully assemble it with clips, screws or whatever, the bead would compress to give a repeatable good seal on future occasions.

Or cut the nozzle so that you form an O ring of small cross-section all the way around. Wait for it to cure then assemble with clips, screws etc.

Hi Sigmund,

I did promote the technique of using silicon for a gasket a while back.
The method I have used a number of times on industrial electrical enclosures was to lightly oil or grease one surface, apply the silicon to the other surface, then press the two components together to evenly distribute the silicon. Only tighten the screws fully after the silicon has cured. Has worked each time. If need be, to avoid dust sticking, any remaining lubricant can be removed after the silicon has cured.

I have not tried the method on decaying pinchweld. Seems to me that applying silicon to anything that is decaying may be temporary at best.
A bond to rubber with any adhesive is greatly improved if the rubber surface is thoroughly cleaned with acetone first.

We do it all the time to provide a seal between panels on aircraft.

Basically it is clean side one that you want the sealant to stick to and clean and also lubricate the side two you don't want it to stick to.

Apply sealant to the side one, assemble and allow to cure.

The problem with your application is that most commercial sealants air cure, so will take forever to harden,

and

air boxes generally have spring loaded clips or a wing nut so that they provide some tension against the seal when closed to keep the water out.

Not insurmountable though. You may be able to get some polysulphide or polyurethane sealant that is a 2 part product, so it chemically cures rather than dries, or just use the single part product and stick it in the corner of the shed for a few days.

The clips though, you could either put the sealant on and only 1/2-3/4 tension them down, or put some spacers in where the seal goes to set the spacing and then repair the small sections when you are done. I'd go with the 1/2-3/4 tension option myself, then put it somewhere where nothing will get dropped on it.

Even with lube, separating the 2 parts can be fun. Even poor adhesion over a large area can make it a struggle.

Once it is all cured, trim the seal to provide a neat appearance and then dust it with talc. The talc takes any residual tackiness away.

Good luck.

What vehicle is it your having problems with an older 4wd or recent model. Is the factory seal worn maybe done up to tight warping the lid and creating a gap.