Saturday, July 7, 2007

De-virginizing the HVLP sprayer

My new HVLP sprayer arrived on Friday -- woohoo! Then on the same day, I got some email back from Alex, the Alexseal rep - he said that Alexseal does not recommend using HVLP to apply their topcoat, as the lower pressure just doesn't atomize the paint very well. (What - now you tell me?!?) This was a bit of a shocker, as their topcoat literature actually specifies the amount of coverage you can expect using HVLP (but it doesn't give HVLP-specific setup instructions). Anyway, I'm supposed to call him on Monday to discuss things in more detail.

After getting that email from Alex, I was a bit worried and went into Internet Research Overdrive (aka google search) looking for references\articles\stories of folks who have sprayed Alexseal with a HVLP sprayer. There's not much out there, at least anything with specific data. (I was looking especially for stuff like specific nozzle and air-cap sizes, spraying distance, reducer speed, etc). The best stuff I could find was from a guy named Tim Lackey, who runs a boat restoration business in Maine. Tim's focus is an older monohull design, but paint is paint and it doesn't care what kind of boat it is sprayed on. There is a lot of good info in his restoration diaries (example), and he also owns a forum that when searched (try "HVLP" or "Alexseal") comes up with lots of good info.

Here's a summary of the advice I gleaned from Tim's site and forum:
  • The biggest issue with HVLP seems to be that it produces hotter air which will tend to cause the paint to flash off quickly. Remedies include: longer hose (allows air to cool down a bit); run part of the hose through an ice-water bath; use a slower reducer than would normally be called for.
  • Due to the lower working pressure of HVLP, Tim suggested adding 10-15% reduction to the manufacturer-suggested amount. This will thin it out more and allow it atomize more easily.
  • The first coat should be very thin; then build it up with the second and third coats (this seems to be standard painting advice, regardless of whether its HVLP or not).

With the rest of the family off at a concert this evening, this was a good time for me to break in the new HVLP sprayer which arrived yesterday. Here's the parts that comprise the system:

I also paid extra to get a disposable cup system made by 3M:

This cup system uses a flexible plastic liner inside of the outer hard cup shell; when you first start spraying, it takes a few seconds to suck all of the air out of the liner, then you're spraying nothing but the paint. The advantage is that it is easier to clean up (just throw away the liner), and you can spray at any angle including upside down.

My wife had suggested that I use some cheap latex paint to learn on, but I had second thoughts on that. Why not try to learn using the real thing, (other than the 10:1 difference in expense)? So I mixed up a small batch: 4 oz of base plus 4 oz of converter. The Alexseal literature says to reduce it by 30-35%, but based on Tim's advice above, I pushed to 40% (3.2oz of reducer, as best I could measure it). I then put the turbine unit outside, hung some cardboard on the inside of the garage door, uncoiled the (stiff) new hoses, got everything ready, tested the mask, and finally attached the gun and gave it a whirl (see cardboard behind the gun, in the picture above). I also painted one of my access hatch cut-out scraps (it wasn't sanded down to 320-400 yet, but I was curious to see the paint on something better than cardboard):

My first impression is that I have a lot of learning to do, to be able to produce a quality paint job.

Thoughts beyond that:

  • There is a lot of stuff going on: trigger control; holding the gun at the right distance from the surface; holding the gun at right angles to the surface; starting\stopping each pass correctly. For best results, all of this needs to be performed instinctively.
  • To try to get a thin coat, I was instinctively pulling the gun farther back from the surface; need to stop that and play with the air and fluid adjustments instead.
  • The turbine unit is wake-the-dead loud! I think I will get some earplugs.
  • Having a fresh-air supplied mask is awesome -- very nice, easy breathing, and no fogging of the face plate.
  • Dragging two stiff hoses around sucks. Unavoidable of course, but this will probably also need some practice, to avoid bumping the hoses into the paint.
  • I am not so sure that I need extra reducer when using my system - the paint seemed awfully runny. Tim's unit (a Showtime 90) is a 2-stage turbine, and produces "only" 5 psi; my unit is a 4-stage and produces 8 psi. My guess (hope) is that the extra psi of my unit will reduce (punny) the need for more reducer.
  • Lots of experimentation will be needed to finetune my skills.
  • I wish I had sprayed all of my primer coats - I could have used all of that practice.
  • My paint color choice looks nice. :)

Cleanup of the gun was straight-forward. I was worried that I'd be scrubbing tiny holes for hours, but it was trivial: fill paint cup with some reducer, spray until clear, remove cup, remove air cap and nozzle, wash them with reducer, wash outside of gun. At least that's what the instructions say to do, so that's what I did.

Lest you think I'm discouraged after all of the above, rest assured I'm not. For my first time ever holding a spray gun, I feel like I did okay. The rest will come with practice. And if I screw up a paint job on one of my floats...well, I may not know how to spray very well, but I sure do know how to sand (so I can sand it down and start over). :)

The other thing that's happened, is that both floats are now sanded down to 320 grit. Fisheries Supply didn't have 400 grit longboard paper in stock, so I had to order some from Noah's to finish the job. Once I have it, I should be all done in one evening. At 320, the floats feel like smooth glass and have gotten a lot of their shininess back.

However, I am concerned about one thing that I need to ask Alex about when I talk to him: in several small spots, I ended up sanding through the primer coats down to the fairing compound. Granted, these spots are all very smooth and feel no different from the surrounding primer'ed areas, but will they look different once painted? Probably I also sanded through the finish primer down to the high-build in a few areas, but those spots are harder to detect. I will be a bit upset if Alex tells me I should re-prime and sand. I'd have to scuff the surfaces with 100-150 just for the primer to stick! :)


Tor Rabe said...

I've been told that the fairing compound, consisting of a large bit of air (in micro balloons), is susceptible to soak water as the LP isn't that watertight. I would be cautious to have any of those areas under the water line as it is even more work to re paint the whole hull in two years than to prime another layer now.....

Anonymous said...

The disposable 3M paint system should be available through Automotive Aftermarket Trade suppliers (that is, suppliers to the car panel beating shops or auto smash repair businesses). In the trade it's called "PPS" (at least it is in OZ and the UK). I mention this as you can often save heaps buy buying from trade outlets as opposed to retail.

Also, re supplying air to your spray gun and your helmet: it is possible (in OZ anyway) to buy a waist belt harness that allows a single input line of breathable air to be plugged into it. This air is then split into 2 via a "T" or "Y" fitting with one lot of air going to your respirator and the other going to the spray gun.
This way you are only dragging around one air line.
Obviously you need a compressor big enough and the air must be filtered enough to be breathable.
Great job by the way!.
(I don't have a google email account but can be contacted off line at:

Jay said...

Hi Daniel,

Yes, 3M calls this the "PPS" system over here in the US too. Thanks for the tip on saving money - I bought the system from the same place I got the turbine and sprayer, and didn't quite realize how pricey the disposable components are.

Regarding using a "Y" fitting to feed both the gun and the mask: unfortunately, I think that would only work with a conversion HVLP gun, ie one that is fed by a full-blown air compressor, not a turbine like my system.

My system did come with a handy-dandy belt that I can wear, and the hoses have belt clips for hooking on. I also plan to use cable-ties to strap the two hoses together along their entire length, so they're not quite so unwieldy.

Thanks again for the advice, and the compliment.