B25 PVC board question

[John Parker]

Finally re-installing my AL fuel tank mentioned in prior post a while back.
Leaked last year as previous owner installation had it against rotting plywood - corrosion hole being the result.
Now the tank is epoxy coated. Going on a bed of epoxy coated marine ply - after removing a good deal plywood rot.
Held off the plywood with 3/4 inch PVC board strips to be attached to the tank with marine sealant.
Question is choice of sealant and surface prep. Sika 291 - 292 ? Surface prep using their products recommended.
3m 4200 - 5200 - don't see any surface prep recommendation for the PVC board.

Recommendations?

[CaptPatrick]

John,

The main surface prep for the PVC foam board is to heavily abrate the faces of the strips so that whatever adhesive you use has a good tooth to grip to. For this application the choice of urethane adhesive is what ever is convrnient and cost effective to you. Liquid nails would even work fine here...

[John Parker]

Patrick,

I remember from when we talked on the phone a while back you had said 4200 or 5200 would be fine.
The trouble is that in doing research, like searching this board, I come up with conflicting recommendations that make it seem like one choice or the other for PVC board is much better and even some that say the choice of which pvc board is important, so I thought I would throw the question out there.

My only criteria are that there be a good seal with the tank, that it stays attached as it will be the only thing holding the strips in place, and that there remain some flex between the tank and the strips to absorb some irregularity in the mounting ( the hull just is not even and worried about stresses transferred to the tank ).

This is the only time I have had a few days in row in the same location as the boat, when I could devote myself to it, so hope to be finished by the weekend if I do not run into unforeseen problems.

Since I have you, a couple of additional questions.
Saw your recommendation for using PVC board to replace marine ply in seat backs and bottoms. This is another project I am in the middle of, and about to epoxy seal a set of marine ply seat backs and bottoms. The originals were uncoated from Pompanette and only now getting soft. I had not considered PVC board as its much more flexible than plywood ( some info from your article PVC Foam Board as an Alternative to Plywood ) and thought this application needed the strength/rigidity
Also considering replacing the boxes the seats rest on with PVC. The current ones are covered ply that is delaminating - and also not up to my wife's aesthetics.

Thanks,
John

[CaptPatrick]

John,

Since you're adhering the strips to plywood, you could throw in a few screws per strip for piece of mind. I'd have opted for Coosa board as a base rather than plywood, but it looks as though you're already committed on that account.

In the case of upholstered items, you can regain some of the plywood rigidity simply by using thicker PVC foam board. Most commercial factory items have 1/2" plywood as the base. Go to 3/4" PVC foam board and you'll have plenty of rigidity...

[John Parker]

Tank -
Flat part of tank bottom rests on plywood. Outside approx. 11" of the tank on either side angle up, though at an angle a few degrees off the hull angle, and previously rested right on the hull- with only soft rubber 1/4 strips in between. I'm using the PVC board strips, after some angle cutting, to make up for the difference in tank and hull angle and the fact the hull V angle changes slightly as you move forward, whereas the tank angle stays the same. So using the sealant to take up some of the irregularity and provide some flex as I know the hull moves and is going to move in relation to the tank. Three longitudinal strips under the center of the tank, and one on each of the outer edges, bonded to the tank with the sealant - not planning any attachment to the hull though its an option.

Seats -
Will have to look at the strength issue. Original seat backs and bottoms were 1" Marine Ply, so for similar strength might have to go with 1.5" PVC ??
How does screw holding compare?

Surface prep -
On 4200 - 5200 note says not to use alcohol to do clean prep as will retard curing. Paint thinner as alternative?

John