I need to build a bathroom vanity concrete countertop for a client. They have choosen an oval undermount sink. I’m looking for advice on how to build the sink mold but more importantly how to release it (it will be cast-in-mold). I’m assuming I will build the sink mold from plywood. But what is the best way to build it so it will release easily?
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I line my molds with gloss formica.
For undermount sinks, I line the outside perimeter of the sink cutout with a strip of 2" high formica (2" being the thickness of the final slab) so the exposed inside face of the final slab will have a perfect surface.
For an oval, I construct the basic oval shape of the form either from built-up 2-by stock or from several layers of plywood. Since I'm going to line the outside face of this oval with formica, reduce the major and minor axis of the oval by the thickness of the soon-to-be-applied formica.
Once the oval is done, I cut it into segments. In this example, I'll say four segments. Sometimes you need more, it depends on the shape and size of the cutout. Realize that the saw kerf will remove some stock, so after cutting it into segments I lay it out with the four pieces in relation to one another and gap the pieces the width of the saw kerf. This way even though the four pieces of the oval are gapped, it's back to its original, pre-segmented size.
I then lock it into this size by screwing cleats to the four pieces of the oval, the cleats hold the pieces so the oval is back to its original size.
Now use contact cement to attach the formica to the ouside face of the oval form.
Screw the form to the bed of your mold, screwing through the built-up ply or 2-by. Position it perfectly.
After the concrete has cured and it's time to remove the oval form, unscrew the cleats. The gaps between the four pieces that make up the oval allow a little wiggle room so you can usually pull or pry the four segements out. You may have to fight the formica a bit, I sometimes score it with a utility knife slid between the gaps, this allows the formica to snap, allowing for easier removal.
Thanks. I was thinking of something similar. I was wondering about a thin strip of foam-maybe 1/8"-wrapped around the form, but I not sure what kind of results I would get.
I do that (wrap with foam) to dowels that I install to make the holes for plumbing fixtures...faucet, soap dispenser, etc.
But since you want the inside face of the concrete cutout to be perfect, the foam would give a very irregular texture to the concrete. The formica will give you a glass-smooth finish on the concrete, that way the finish on the inside face matches the finish on the top of the countertop.
Have you ever tried using styrofoam as the mold? With care, it can be cut/sanded very smooth then covered with vinyl tape.Jeff
No.
Thank you for your post. That and others on the subject have been very helpful in making a decision to use concrete counters in our new house. It's probably not really a DIY job, but we're gonna try. Worst case - we chuck it and buy something else.
Jon
Best advice I can give is to decide if you want to form and pour it in place, and trowel your finished surface...or if you want to cast it inverted.
The latter is much easier in terms of getting a great looking countertop, but then you do have to install it.
Mix your own concrete. Don;t use bagged redimix like quikrete. Control the mix and you control the outcome.
Do samples. On a small scale these will allow you to fine tune your methods. Everythng from coloring, to finished surface, to sealer. It's much better to screw up a sample that can later be used as a stepping stone than to scew up a 300# chunk of concrete.
Have fun!
Damn! Everytime I finally let go of the idea of casting my own counter I read something like this and get excited again.
Question: What do you mean by casting them inverted? Or perhaps I mean why cast them inverted? I have a lot better chance of doing this if I'm not in the way of my wife in the kitchen. Thanks.
Make a mold the size and shape of your desired countertop. Remember, you;re casting the slab upside-down, so build the form backwards, so to speak. Essentially a flat surface with a 2" (or whatever rim for thickness) to contain the mix. I line my forms with cheap gloss formica. I buy the damaged sheets at the home center for half price. Place the mix, reinforcement, etc, etc...let if cure...pop it out...
viola!
Okay, it's a bit more complicated than that...but you get fabulous results.
Mirror smooth finish on the slab, you can use molding in the form to get a decorative edge, and you don't have to clutter up the kitchen while you're doing this.
Plus, no troweling to try to get a smooth surface, and no grinding to try to smooth the surface that you tried to trowel smooth.
<Sorry Mad Dog...>
I may find the time to try some samples soon. Thanks.
Thanks to everyone for their input. Has anyone tried the Cheng Concrete product NeoMix--it has all the additives already in it? You mix it with high-strength bagged concrete. Also, Mongo, are saying that by using gloss laminate instead of the white melamine to make your forms you don't have to do any grinding? That would be a huge time and money saver.
No grinding at all with formica-lined molds.
When I was figuring all this stuff out over 10 years ago, I tried many, many things for the form. Melamine was one, but I only thought it worked okay. There are a few things that work better, and formica is one of them.
When I take a slab out of the form, you simply have to understand that it's like looking in a mirror. A lot of times I have to take the surface down (standard grit on a palm sander) just to take it down to a less reflective matte finish.
The surface is still dead-flat and perfect...but now it's matte instead of gloss.
Depends on what level you want.
I've never used Cheng's mixes.
I've only ever used melamine but I'm going to have to try formica. What's the best way to cut long 2" strips of formica? Thanks for all your help.
I contact cement a wider, full-length strip to 2-by stock, then rip them (formica side facing up) to the desired rim height on the tablesaw.
A 2" thick slab is my standard.
The rim pieces can be reused multiple times.
You're most welcome.
If you don't mind, I'd like to pick your brain one more time (before my Cheng book arrives...)
When casting - how do you make sure that the bottom of your form (which will be the top of the counter once done) is dead flat? Do you cast on a concrete slab, or a workbench top or what? I'm concerned about irregularities in the form telegraphing to the finished product. I'd assume that the gloss formica you're using is cemented to a flat substrate such as MDF, but you still need to keep that as flat as possible, right? Would a basement concrete slab be sufficient?
Thanks for all your helpful advice. This looks to be fun.
Jon
For the first countertop I made, I made a grid by half-lapping 2x4 kiln dried stock to essentially make a large, grid-like tabletop. Put a couple of strongbacks on it for added strength.
By itself, that was as flat as flat could be. That was placed on sawhorse-type supports that I made to hold the grid, the whole thing was leveled. Use winding sticks.
I simply tossed a sheet of 3/4" ply on the lapped grid and the formica goes on top of that...no cement, no fasteners, it floats. Draw the layout lines...then place the rim pieces in place, and screw up from below, from between the grid pieces. From the bottom screw through the ply/formica and up into the 2x piece that makes the rim. You can first run a bead of caulk if you choose, sandwich that between the rim and the base. That way no cream from the mix will leak out from the form.
I had leakage from one of my samples that lead me to caulk when I first started doing these, but most everytime I now add molding to the inside of the form, and when that is detailed no liquid will leak from the form. A learning curve for me, if you will.
Once a few screws are set in each rim, I can slide the mold over the edge of the grid and screw the rim every 6" or so.
On several occasions I've transported the slab in the mold. I've screwed dolly wheels to the long side of the form...the 2x rim piece...and after the slab has cured I can flip the slab/form up on edge and wheel it to where it needs to be.
Lean the assembly against the countertop, then unscrew the form, then simply flip the slab up on the countertops. It has allowed me to set #450 pound slabs myself.
Mongo,
I'm thinking of building concrete countertops, and I've got to say I've learned so much from reading your informative posts! Many thanks, and keep the info coming!
-Kyle
Kyle,
I can't send attachments through Taunton, but if you want more info email me @ MongoSpecs "at" cs.com (replace "at" with "@") using "concrete countertops" in the subject line.
Mongo
Get the book Concrete Countertops by Fu Tung Chen. It's a Taunton publication. Read it and your knowledge base will increase greatly.For what it's worth: I was casting countertops and other things long before the book came out and I still find it helpful.Jeff
Believe me, it will be fun - even a chance to experiment. I was one of those weird guys in the civil engineering program in college that built a concrete canoe. My class, it sank (we broke it), but hey, there was a lot to be learned. I've been considering lightweight aggregates, coloring, different strengthening admixtures, and I really like the idea of inverted casting on gloss formica. Installation can't be any worse than putting in granite, and I just have to remember to cast in nailers for the dishwasher.
One question - I already have a kitchen sink for the new house - it's a rather large single bowl Advance Tabco stainless sink. Undermount style. How do you attach an undermount sink to concrete so that it doesn't fall off someday?
I also have concerns about durability and stain resistance. How does one approach those issues?
With a lightweight aggregate you can reduce the weight of a 2" thick slab from about 25#/sqft to 18. There are pro's and con's to using them.
Undermount sinks can be hung myriad ways. They can be hung right off the slab. Directly with tapcons, or screwed into brass or wood inserts that were cast into the underside of the slab when forming it up. If using tapcons, be careful how closely you come to the face of the cutout. Yu don;t want to pop a chunk of concrete off your slab.
Undermounts can also be hung off the side walls of the sink cabinet. There are commercially made hardware kits that you can use, or you can fabricate your own, often quite easily, just using a couple of wood cleats. It all depends on teh sink, the cabinet...etc, etc.
There are two types of sealers...penetrating and film sealers.
If the countertop is to be used in a working kitchen...hot stuff from the cooktop or oven, cold stuff from the fridge or freezer...I prefer a penetrating finish.
If film sealers are exposed to big temp swings, the film can discolor or even be lifted right off the slab. It's kind of like a piolyurethane film over wood...you damage the film you really just can't patch it...you have to r&r it to make it look good again.
There is also a bit of surface prep work required to PROPERLY APPLY a cured film sealer. Most people despise the prep work and only want to get things done, as such, I've seen many DIY-applied film sealers fail.
The good thing about film seales is that they are near bullet-proof. Water, citrus, etc. Again, think poly on wood. The bad thing? You have to be somewhat careful what you put on them (temperature) and they look somewhat synthetic. Again, it's a film over concrete.
Penetrating sealers are much easier to apply, but they do need to be renewed. They're of the "wipe on, wipe off" family. Takes about 30 minuted to apply three coats. I then like a coat of paste wax.
The paste wax is an additional layer of protection, so to speak. You can renew the paste wax as required...maybe once every one or two months. The penetrating sealer? If you wanted to, you could reapply that every several years as required.
Penetrating sealers allow you to use the countertop for hot things, for cold things, etc. But it is more prone to being allowing things to stain the countertop.
You can protect the countertop from staining by controlling your mix when you make your concrete. Also, any surface etching duer to citrus con be buffed away, or just let it sit and it disappears with time.
Think of concrete in some ways as you would a wood countertop. Don't let spilled liquids sit on it overnight, get into the habit of a simple wipe-down whe you clean up.
There certainly are pro's and con's to using concrete. Just as there are with wood, granite, marble, tile, stainless...etc. You simply have to match your personality (as well as that of your spouse) with the material.
See if this attachment works:
Oops...didn't mean for it to be that big...<sorry, Mad Dog>
The black perimeter is the formica.
The gold is the four segments of the built-up oval, the black lines show where it has been segmented into four pieces.
The yellow "dominos" are the cleats, the black dots are screws that hold the cleats to the segments, which in turn secure the segments in place.
Hope all that makes sense...<sorry Mad Dog>
View Image
Edited 3/2/2005 2:00 pm ET by Mongo
A few more clicks and ya coulda posted it full size...LOL
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