No need to sand a 'new' barrier coat to make it smooth.
You can apply the first coat with a roller but successive coats with a large polyethylene trowel (not a roller) and 'fill' the previous hills and valleys from the roller application. Method: roll on and immediately 'wipe' with a trowel, the semi-hard 'peaks' from the first 'roller coat' will set the height for the trowel to 'glide' on --- thus quickly filling and fairing the 'valleys'. This 'troweling' is in the same manner as applying the final gelcoat to a MALE plug mold. For perfectly smooth and faired you can finish with a thinned out fairing coat using a BIG trowel.... and the WHOLE job should be continuous so that NO layer 'fully cures' so that there is continuous bonding throughout. This can include the first bottom paint coats (hot-coating) for much better 'adhesion'.
The MOST IMPORTANT characteristic or goal of ANY barrier coat system is the MIL-THICKNESS. Without the proper mil-thickness you are vulnerable to future water (principally water vapor) permeation through the barrier coat into the FRG. With a roller you have to apply MORE barrier so that the deepest 'valley' is at the proper thickness; with a smooth or troweled-on application that required thickness is more easily controlled.
How to know that you have the proper thickness --- go to an industrial paint/coating supply and get 'mil-thickness gages'. These are 'little comb-like or 'toothed' gages' that you 'push' into the fresh/wet paint or coating and if you get discontinuous dots or dashes in the fresh surface instead of lines the thickness is too small and you need to add more paint/coating. With solvent based coatings you need to calculate the % solids so that when the solvents 'flash off' and the coating 'shrinks' you ultimately arrive at the recommended 'dry' mil thickness not the 'wet' / 'as applied' mil thickness.
Each barrier coating mfg. lists the proper 'thickness' in their 'tech manuals'. If you dont apply to the recommended (dry) mil thickness, you can expect to not have sufficient barrier applied and you will/may/can after some time observe the 'return' of the blisters/pimples. I always apply more thickness than what the mfg. recommends as I dont want to do a job 'twice' ... and barrier coating manufacturers seem to always continually 'increase' their thickness recommendations over time.
hope this helps.
Last edited by RichH; 02-09-2011 at 11:45 AM.