Supporting a Molded Plastic Sink

Suggestions: wooden supports or a mortar bed. January 21, 2007

I built a rather sturdy cabinet system to house (among other things) a 6' long ABS plastic convertible (kitchen/darkroom) sink assembly. It's un-reinforced, and I'm trying to get the sink as strong/rigid/problem-free as possible. I've already epoxied birch ply strips to the recesses (ribs) at the bottom of the sink with the hope of strengthening it. The whole thing is vacuum-molded... and the lip is totally hollow. However, this thing needs to support quite a lot of weight, and I don't want to risk having the structure of the lips sag. So I was thinking perhaps I should build up the undersides of the lips more substantially (perhaps with glass and cloth or wood and bondo or something). Any suggestions? Also, how should I seal the edge? Silicone? The counter it's being built into is maple ply/9x layers lacquer.

Forum Responses
(Cabinetmaking Forum)
From contributor J:
I would guess your best bet may be the simplest - build supports under the bottom of the sink, similar to the cradle you would build for a cast iron farmer's sink. That way you won't have to worry about the weight on the rim.

From the original questioner:
Yes, I've already done that... birch ply ribs (1.5" deep) along the length of the thing and those will rest on 2x4 cross members. I suppose those should take up the slack. Any recommendations for sealing the rim? I really can't think of anything that would work with a flexible rim like that and look decent at the same time.

From contributor L:
It's been a long time since I've had my hands wet, but maybe this will help... The support structure contributor J suggests will be best. Making the ribs perpendicular to the sink's rib-structure would be stronger still.

As far as caulk, whenever I've got a problem surface, I use Sikaflex. Just make sure to use gloves and have some acetone around for clean up. The thing I'm most concerned about is your countertop. Photo chemistry will make a mess of that in record time. You might consider p-lam instead of lacquered plywood. Of course, you know the inherent problems of having your darkroom double as a kitchen? Don't forget an exhaust fan that is light baffled.

From the original questioner:
Well - response appreciated. I'm an architect and do a lot of structural work. I'm quite satisfied with the structural solution. Just not with the lips so much. I feel my solution is good for (direct reinforcing) instead of running members crosswise and allowing the surface of the sink to flex like you were suggesting. So that's not going to change.

I suspect the darkroom chemicals (black and white only) will not interact too badly with the lacquer (we're talking about soda-water here) - though most of the processing should occur in drums anyway. Hopefully there won't be too much splashing around. Though perhaps I should have considered an epoxy surface.

Thanks for the Sikaflex tip. Is it colored? I'm just concerned with the sealant being visible.

From contributor D:
When setting fiberglass subs, I've always bedded it in a big wet pile of plaster. When the plaster sets, you have a rock hard custom molded support system and it never breaks down or crumbles. People have been doing it this way for years and years as far as I know. Not sure if this is feasible in your situation, but I'm quite sure it would work if your cabinet box can be designed to accommodate the plaster.