I am building a line of Aluminium "general purpose" motor boats.  What abrasives do you recommend for blending hull welds?
For blending the welds we would recommend the SMT Flap discs.  If it's hard aluminum the SMT627 should work fine, if it's soft try the SMT630. If the welds are too large for these discs to take down in a timely manner, do the brunt of the take down work with the A46N Supra regular grinding disc and then go to the flap discs for the actual "blending" portion of the work. 
 
After the welds are blended, you could use either the regular PS33 or the non-woven discs to try and even out the entire surface.  The grit would be determined on what product your working on, but 80 grit would be a good place to start with the PS33 and perhaps the medium density on the non-woven.