I am installing a jacuzzi whirlpool drop-in tub in my bathroom. The tub is a 32 x 60 that is sold at Lowes. I called jacuzzi and they said I could drill holes and install the faucet (and handspray) in the triangular flat areas in each corner of the tub.

The problem I am having is that the underside of these flat areas is not flat, there are ripples and angles and built-up areas. When I put the fixture parts into the holes, the washer from the bottom (about 2" Diameter) does not lay flat, which would tilt the fixture. The worst gap from one side of the washer to the other is about 1/8".

What is the proper procedure for dealing with this? I can either sand off the high part until its flat or add some material to build up the low area until flat.

What do the pros do when they have to deal with this?

Thanks
Steve