[TR] pressure sandblaster tips?

[Gary Nafziger]

Cleaning up tr-3 fenders:  I purchased a harbor freight pressure blaster and within half hour ruined the nozzle last summer (ball valve just wore out by sand going through it).  I wasn't really happy with the blaster as it seemed to work intermittedly.  It would  blow very well but then seem to get plugged or almost plugged at times.  I've since replaced the valve with a deadmans valve but when first using it I got sand blowing out with minimal pressure.  Not nearly enough pressure to be effective.  I took everything apart and all lines were clear except for the sand hose from the bottom of the blaster to the nozzle which was full of sand. 
I have a 4 hp compressor with 25 gallon tank.  It does 9 scfm at 40lbs and 7 at 90.  I know its somewhat small for pressure blasting but think it should do better than that as it does handle my cabinet blaster ok.  The pressure blaster is harbor freight 40lbs.
My understanding/experience with pressure blasters is minimal.  My main question is with the sand hose from blaster to nozzle.  It does have a valve right under the blaster which can control sand flow.  Should this be wide open or partially open?  I'm thinking if its wide open then too much sand is filling the hose resulting in plugging or partially plugging the hose.  But what will it help to partially close the sand valve?................. because when I shut off the nozzle to re-arrange the fender or do another task................. won't the sand continue running into the hose anyway.........potentially plugging.
Just curious if anyone else has had this problem or if anyone else has more extensive experience with pressure blasting.
I'm basically cleaning up the edges/corners of fenders and might just resort to hand sanding and POR-15.LOL

this might be borderline british so sorry!
gary n.