Today there are much better alternatives (two part, water based) that should be investigated for they only require that you roll it on - as many coats as needed to fill - and then just sand it once to a final finish before painting (or flying).

 

I used three coats of WLS back in '94-'95 and was pleased.  Each coat required one week of prep (paper on the floor, plastic covering everything else, surface cleaning), 20 minutes to spray and two weeks of wet sanding before the next coat. 

 

I mask sprayed Krylon N-numbers on the side and after certification flew the plane in primer for a year before it was painted. 

 

One problem that the primer surface had was that it was extremely conducive to the formation of static - so much so that running my hand over the cowl would cause the hair to stand up on my arm (Loran would never work once airborne, forcing me to switch to GPS before its time).

 

After removal of the Krylon and a final light sanding, the plane was painted (PPG automotive base/clear) - lo and behold, the Krylon number area was swollen and, in the right light, one could make out the old numbers.  It took about 6 months for the swelling to disappear.  Today, in fluorescent light, I can see the weave in the e-glass cloth underlying the paint and primer (disappears in sunlight).

 

If I was prepping today, I would never use WLS.  Times have changed.
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I agree with Scott 99% of the time but my experience with WLS is quite different than his.

I have been using WLS primer since the mid ‘80s when we were using the WLS paint system on IMSA GT race cars because it was extremely tough and easier to apply than Imron. I believe I was the first guy to introduce the product to the Lancair builders in the very early days of this list. My IV-P was topcoat painted in ’95, still looks great and is frequently complimented on its appearance.

Primer is NOT a topcoat. It should not be used as a topcoat. It is designed to bond to a topcoat so you should not be surprised if it bonds to whatever is applied over it, be it “”rattle can” pint, oil or bug splats. If you want to fly before the final topcoat then apply a thin, cheap one stage topcoat that you will sand off later.

Paint is heavy! If you put it on with a trowel you will have pounds of extra weight. Spray one day and sand the next for the first couple of coats. If it doesn't cure overnight then it is defective or wasn't mixed properly or sufficiently. Spot prime areas that need lots of work. Stop sanding when you burn down to carbon. If you keep sanding you will only make things worse.

You do not need to, nor should you, wet sand the primer. The surfactants, minerals and other contamination in, or the water itself, will bond to the primer. I used guide coat (thin black fast dry paint) and 180 grit to get the surface flat followed by guide coat (again) and 220, then 320, then 400 grit to progressively to remove scratches. Take three welding rods or straightened wire coat hangers,  bend them in half and wrap tape for 6" around the cut ends. Use this like a mini carpet beater to clean your sanding media of clogs.

After you are happy with the flatness and finish, spray on a thin reduced primer coat with a couple drops of pigment in it so you can remove it later with precision.

If the weave starts to show it is the fault of the substrate, not the primer. The heat and pressure that is used in the composite curing process induces stresses that relieve themselves over time through microstrain. This process can be accelerated (then sanded flat again) by thermally cycling the primed parts. I would spray some guide coat on the various parts so they are relative dark in color, put them on supports and place them in the summer sun. Repeat daily, weather permitting.

The best tip I can give for producing the best finish coat is to NOT do it yourself. Topcoat application is a skill that requires practice and is best left to someone that does it on a daily basis. Find a reputable automotive body shop with a large booth. IMHO you can get a better and cheaper job at an automotive shop versus and aircraft specialist if you take the time to explain how to handle, and the importance of, the various parts. A picture of a completed aircraft is helpful. Let them do the final sanding and prep. Hire a pinstripe specialist to do the graphics taping. It takes an experienced eye to put a tape line on the 3D surface and have it look good from all angles. I have seen lots of  botched trim lines on the ramp. They draw the eye like a bad toupee. Gee, your paint job looks...errrr...ahhh... fast! Yeah!

Most importantly, take the painter and his assistant aside and give each $50 with the instruction to paint the plane like it belonged to them. The fact that you trust them enough to give them a bonus up front will make you a freaking hero! Promise another bonus at the end of the job too. Be on site for the actual painting to help move parts, mask, clean, run for lunch or whatever else is needed. A little humility and respect will be rewarded with a quality job that lasts for decades.

This works. I have painted two IV-Ps this way, both won outstanding workmanship awards at OSH and both look great a decade later.

BTW, the WLS reducer is way too expensive to use for cleanup. Use some el-cheapo fast lacquer thinner for that.

Regards
Brent Regan