Workmate table top
I wanted to build a worktop to be clamped into a workmate or folding workbench to provide a larger working area and as I had some spare 18mm MDF, 2×2″ and 2×3″ CLS I used them. MDF can and does warp so I framed the edges with the 2×3″ which I glued and clamped and then drove a few screws to hold in place. This will provide extra strength and also provide a surface to clamp against.
For additional strength I added three 2×2″ cross ribs these were glued and I stood heavy paint tins to hold them in place as the glue dried.
This when paired with two sawhorses would be fine, but as I wanted to be able to clamp the top, so I added a length of 2×4″ screwed into the ribs for the wormate to clamp to. I did not glue this piece so if I want to use the top with sawhorses this can just be quickly removed.
Middle rib clamped into the workmaye/folding workbench
To finish the top and protect it, I followed the same procedure as for my workbench, I thinned oil based polyurethane with white spirit 50:50 for the first coat and second coat. I really let it soak into the MDF, the third coat followed at around 75% varnish to 25% white spirit and the final coat of just varnish, I sanded with 240 prior to the final coat.
Polyurethane does not need to be keyed between coats, I just sand before the top coat to ensure a nice smooth surface. The reason for thinning is that the varnish will soak into the board and protect it rather than just applying a top coat of varnish that could just chip off. It has worked really well for my bench top the only issue really being the drying time and the smell (as its oil based rather than water based). To finish I also gave the underside of the board a coat to seal it and to ensure it does not absorb any moisture and warp. Job done.