Mounting External Fixtures to PMF over Ply

[kayak-camper]

Hi,

What's the best way to mount external fixtures to PMF over ply?

I have a roof rack and a canopy but I really don't want water getting in, so I was wondering which sealant would be best. Also, I was thinking of mounting a piece of wood externally and then mounting the canopy and rack to that, i thought it'd mean less holes in the body of the camper.

Thanks

[KennethW]

I would bolt through. Using polyurethane sealant. Between the hardware and the roof.

[kayak-camper]

Thanks, would you do that before or after applying the PMF?

I'm thinking after but would the sealant seal properly on the PMF?

[Tomterrific]

I just mounted a TV bracket for a 19" flat screen to the side of my camper. I drilled through where a batten is located for strength. Here is what I did for water proofing.

1. After drilling the holes I took some leftover polyurethane and used Q-tips to seal the dry wood inside the holes.

2. I used rope caulk, a non drying caulk, rolled into a worm shape around the hole. When the bolt and washer tightened, the caulk squeezed around the threads and out the edge of the washer making a truly watertight seal.

Tt

[working on it]

... After drilling the holes I took some leftover polyurethane and used Q-tips to seal the dry wood inside the holes....

* I use a similar method: drilling a hole, then liberally coating the bolt/screw threads with Loctite PL (or Gorilla Glue Clear Grip Contact Adhesive, if there is no washer to cover the spread of excess adhesive) before insertion. Then, I use stainless fender washers on both sides to make a waterproof patch, sealing the hole, before tightening the fastener. I wipe away the excess, usually.
* So far, with dozens (literally) of holes thru my 3/4" plywood structure, there have been no leaks, with most of the larger openings sealed 7+ years ago. They are periodically tested by thunderstorms every time I camp, so I am constantly observing their status, though I have great confidence that they're sealed for good.