You will need some strips of laminate covered MDF for the sides of the mold and to outline the sink opening. The strips for the sink opening are 2 1/2 inches high and the strips for the edges are 2 1/2 inches plus the thickness of the melamine covered MDF that is the base of the mold. Some laminate covered MDF is 3/4 inch thick others are 13/16 inch and others are 7/8 inch, yet they may all be called 3/4 inch.
Note: Use a fine toothed saw blade to ensure that you do not chip the laminate. Any imperfections in the laminate will show up in the finished concrete countertop.
Before you assemble any of the side strips, spray the cut edges that will be facing up with the adhesive, wait approximately 45 seconds and then place a layer of cellophane tape over the edge and trim it with a razor blade knife to fit. The spray adhesive and cellophane tape will help prevent water from entering the MDF and causing it to swell.
Note: You may wish to cover the top of your workbench with a sheet of masonite to protect the surface from spilt concrete as you are pouring it into the mold.
As previously mentioned, if you have a large countertop you will want to split it into manageable sections. In the case of our example we will divide the concrete countertop into two sections. For ease of explanation, we will divide it in the middle of the countertop, which is not in the middle of the sink, and we will divide it at right angles to the front of the mold.
Note: Many professional concrete countertop manufacturers will divide the concrete countertop on a slight angle, rather than at right angles. They believe that the aesthetics of an angle joint is preferable to a right angle joint. There is nothing wrong with this concept, but it makes the mold somewhat more difficult to construct.
Find the center of the countertop, and make sure that you are not within 1 1/2 inches from any holes, such as the hole for the facet. If you are closer than 1 1/2 inches from a hole, move the divider appropriately and then cut a 3/16 inch deep, thin kerf through the laminate.
Note: You will have to cut a 3/16 inch kerf in the sidewall strips, in line with the kerf on the base of the mold to take the dividers.
The sink is done in a similar manner, but the support strips face the inside, they are where the sink hole will be. The outside strip also needs a kerf to match the kerf in the base of the mold for the dividing strip. Screws should be inserted every 6 to 8 inches along the length.
Note: The support strips that form the outside sink edge of the concrete countertop are rounded over with a 3/4 roundover router bit, as shown in Figure 11. The purpose of the roundover is to give the hole for the sink rounded edges rather than square edges.
Figure 11 - Outside corner of sink hole - 3/4" roundover
These edges, because the laminate has been removed when they were rounded over, must be sealed using the spray adhesive and clear cellophane packing tape. Spray the adhesive on the exposed MDF and wait about 90 seconds. Then apply the cellophane packing tape. Trim off any excess packing tape with a razor blade knife.
A cross section of the finished mold is shown in Figure 12.
Figure 12 - Details of mold construction
The strips should be fastened together at the corners using the silicone caulk adhesive and the 1 1/4 inch drywall screws and countersunk flush with the surface of the melamine. A detailed section of the side wall construction of the concrete countertop mold is shown in Figure 13.
Figure 13 - Details of side wall mold construction
Note: Place enough silicone caulk adhesive so that it oozes out of the joint. Wipe off the excess adhesive with a cloth and alcohol and/or razor blade knife.
How to Make Concrete Countertops - Index