Leo,
I always try to go to the horses mouth so to speak - below are the suggestions from two manufacturers. When I did our counter, it was suggested that I first put down 1/4" MDF glued and screwed. Then having a "new" surface, apply the new laminate as usual. The big reason I was told to go the extra step was to prevent delamination if hot items are placed on the counter. I got this from two different counter guys. Most say it's overkill, but it's worked fine for 15 years of abuse.
Good Luck!!
Wes
Wilsonart:
I want to laminate over melamine or existing laminate can I do this?
We do not recommend bonding to existing laminate or melamine covered surfaces (in fact, our warranty does not cover these applications) because they are non-porous and often prove troublesome. If you decide to try anyway, follow these five guidelines:
1. Sand the entire existing laminate surface with coarse grit sandpaper and then clean the dust off. This creates scratches for the adhesive to grip into.
2. Allow the substrates and the glue to warm up to room temperature before use and perform all bonding at room temperature.
3. Ensure 100% of the area to be bonded is covered with an adequate amount of adhesive (see product label for specific coverage amounts); be especially diligent about coverage around the perimeter.
4. Do not try mating the substrates together too soon. The substrates will be ready to bond when the film of glue on each substrate feels tacky when you firmly press your fingertip onto it. The glue is NOT ready to bond and must dry longer if the glue is still wet or if any glue actually sticks to your fingertip and creates a rubbery “leg” as you pull your finger away.
5. After mating the substrates together, use a hand-held J-roller with as much manual pressure as you can bring to bear over the entire surface. Multiple passes are recommended.
Formica:
http://www.formica.com/publish/site....0031.DownloadFile.File.tmp/4 Resurf Tech.pdf