Understanding Coverage with a Post-Cat Primer

How to estimate the coverage you should get in real life, based on reading the can and measuring the wet thickness you apply. January 29, 2014

I started doing finishing work for the company I work for as a side gig, and I did some numbers on what I'm getting per sq ft, now that the job is pretty much done. According to the ML Campbell rep, I should be getting about 250-300 sq ft out of each gallon, and I'm not getting anywhere near that. I started out putting on 2 coats of primer, ensuring I get the grain of the birch veneer panels completely covered so you don't see it. Then I wondered if I really needed 2 coats, so I switched to one coat a bit heavier. I did that for the rest of the job and now I'm going to need another 1-2 gallons of this primer and possibly another gallon of custom mixed paint, and I'm questioning whether or not I need to completely hide the grain with the primer. I'm using Magnamax post cat lacquer topcoat and have found I only need one coat of topcoat.

Do I need to completely hide the grain with the primer before I put top coat on? (This would yield less material on primer, which would be a cost savings for me.)

I'm paying about 42.00 a gallon for the primer. Is this really the best option for the primer I'm using? When I'm doing side jobs for my own clients, I tell them it's more money for the primer and paint, which freaks them out, so I wonder if there's a cheaper alternative, just as good quality as what I'm currently using.

I spray with a Binks Rapitor AA1500 airless air assist pump gun.

Forum Responses
(Finishing Forum)
From contributor R:
Is this a job you're doing for yourself or is this a job for a customer where they pay for the materials and you do what it takes to produce the job to look exactly like the color sample? If you're bidding a job and you've done your number crunching and your sample took three coats of primer and two coats of color and a coat of clear over the color, the material cost is reflective in your bid.

So to say you are paying $42.00 a gallon is a bit misleading since your customer is the one paying $42.00 for the gallon of material.

As far as completely covering the grain, I would refer back to the signed off color sample. If the grain is filled on the sample I would fill it on the entire project. As far as getting something real cheap that behaves as a more expensive product does, please tell us what it is and where to get it. As far as the S/F per gallon figures, I would ask the supplier how they arrived at that figure and what type of spray gun they used and to what substrate they applied the coating.

Finishing is like baking a cake; if you exactly follow the recipe, your cake should look like the picture in the cookbook. If you cheap out on the amount of cups of flour the recipe calls for and you use one egg instead of two and you use eleven tablespoons of baking powder instead of seven, I would expect the outcome to look different than the sample picture in the cookbook.

If a customer says the price is too high, make another color sample using way less material and real cheapo nonprofessional products so you give them the option to purchase a real nice job or a not so nice job from you. I suppose if you can make a few bucks off the not so nice job, it's better than losing your tail by giving them a great job they don't want to pay for.

From contributor L:
You are misreading the label, plain and simple. The can/specs state that you will get that coverage at 1 mil thickness.

Theoretical coverage at 1 mil dry:
(Coverage figures do not include spray loss. Also allow for surface irregularities and porosity of wood surface to be finished.)
535 sq. ft. per gallon
% Solids - by Volume: 33.38 2.5

This means if you don't thin and you put on a wet coat that is 3 mil, you will be able to get a 535 sq foot coverage at 1 mil dry thickness. If you are using a AAA pump that has an efficiency of 65% for coverage, you are down to 350 sq ft (at 1 mil dry).

Now, you say you are trying to do a grain fill on birch plywood. Birch plywood isn't that smooth as grain goes. Birch has a somewhat open grain structure so you are going to use substantially more primer trying to get a fill. 1mil dry isn't going to cut it. You'll need at least 4mil dry to cover the grain so now you are down to 90sq ft coverage per gallon.

A better primer would be Clawlock, which is MLC's post cat primer. About $5 more per gallon, plus the price of the catalyst. Fills much better, thin at 10% and do 2 coats. Heavy sand on the first coat, light scuff on the 2nd.

From the original questioner:
Thank you! I debated between Clawlock and Magnaclaw and chose Magnaclaw for a few reasons - cheaper price, was told it will work well for my situation, and I wanted to try something new that I didn't have to catalyze, so I wasn't stuck to spraying exactly what I mixed and taking a chance on a lot of wasted, unused material.

I agree that birch isn't the most paint grade friendly material, but it's what the company chose that I'm doing the finishing work for. I got about 128 sq ft per gallon, including overspray, waste from cleaning gun, etc.

I use to have a tool with ridges in it from ML Campbell that measured wet mils, but unfortunately I can't find it right now. Would this tool be the best to see how many mils I'm laying down?

Now, these parts need to be finished on both sides and the edge banded edge, so I stapled sacrificial pieces to the bottom so I can spray both sides at once, so how would I use that tool if the part is vertical?

Any other suggestions on how I can get better sq footage out of a can? Also, when spraying, if I don't put a heavy enough coat on, it gives this kind of orange peel look, but if I spray more or heavier, the finish meshes together and that look goes away, which is a good thin). Is the airless air assist pump I'm using a good quality, in your opinion? What are your thoughts about the new Kremlin?

From contributor D:
What is your sanding schedule? Prior to and in between coats? As far as measuring mil thickness (when you get another tool), mount a dummy piece next to what you are spraying (could even tape a paint stick onto something near the piece being sprayed) and pass over it - same as you do the actual project - and measure your mils on it. Do it a couple of times, and you will develop a feel (by sight) for what it takes to achieve the proper build. Then periodically re-measure as you are spraying to make sure you are still on track. Are you setting your products viscosity to the recommended flow?

From the original questioner:
To be honest, I don't know. I have messed with the settings from where I'm hardly getting any material out to where I'm flowing out too much. Right now I get a good coat and if it leaves that dry orange peel look, I add a bit more so the finish melts together. How do you know you have the right viscosity? And what info from the can do I need for this?

From contributor N:
One place you may be able to save some material is on the two business ends of your spray setup - the tip on the gun and the air and fluid pressure on the pump.

I'm not familiar with ML Campbell products but many here are and no doubt have found what combination of the 3 work best at getting the job done without fogging up the booth and loading up the filters.

From the original questioner:
Thanks. I have my settings where it's putting out enough material to cover properly, so I may just need to reconfigure how much material I need moving forward.

From contributor M:
As far as your first coat, it should be very light, then sand with 220 or 180 pads. Don't worry about sand through. Then spray a heavy coat to cover. Sand with 320 or grey pad. Don't sand through.