I use some Elmer's spray adhesive to glue the carpet to a sub box and 1 week later (on a hot sunny day) the carpet started to unglue itself...

I applied a thick coat on the mdf and carpet, waited until both stick to each other. I used the wrong stuff or I did a bad job?

 

Sounds like you did a decent job - you just used a poor product. Use liquid contact cement or if you want an aerosol, the easiest to find consumer product that is DECENT is 3M Super 77. Home Depot and Rona both should have it.

 

Yeah I used the super 77 stuff. Once it sticks and dries that's pretty much it.

 

What else do u expect it do?

 

I expect it not to come unglued. The 3M stuff seems to hold up to the harsh heat of a vehicle very well.