Oil finish with airless equipment
I used to do new construction homes and would finish all the woodwork nailed on the wall. I had great results. Enameled trim should be all installed first - that way you can caulk any gaps, fill holes and then spray. All I used was airless. For oil base varnish I used a 409 tip. I used BM Impervo for enamel and I think I used a 409 tip. I thinned it a little. I think aa and airless spray fairly alike. The tips in them atomize the material like the air in aa does. Just be consistent, make even passes, overlap your passes and lay it on as heavy as you can without getting any runs.
I find my airless paint sprayer the most difficult to use of the three systems I have (Turbine HVLP, Airless, and Air Assisted Airless). The difficulty comes in the relatively wide spray pattern and the large amount of finish that is dispensed by this type of sprayer. The more finish that is dispensed, the faster you have to move and this can make consistent speed and distance control harder to achieve. These same characteristics are an advantage once you learn how to move with it and develop your techniques. Airless sprayers are great for spraying heavy-bodied finishes like oil and (particularly) latex paints.
To get started, ask your paint supplier what the professional spray painters in your area are using for their "high quality" jobs. Start with that primer and paint and see how you like it. They should also be able to tell you what tip to use for whatever type (viscosity) of finish you intend to use. Generally speaking, I don't think you will have to (or want to) thin any of the higher quality paints so long as you have the proper tip. The Titan 440 is a very popular pump (of the professional painters) in this area and our Sherwin Williams stores sell this brand. Producing a nice surface with airless sprayers starts with the proper tip for the desired paint and ends with laying the paint on as even and wet as possible without runs.
The pace and distance (from the surface) at which you move the gun will allow you to control how much finish gets applied to any given surface area, all else being equal (tip, finish viscosity, etc.). You will need to overlap each line of spray by about 1/3 the width of a single line. So, if your fan pattern produces a 12" line, you would overlap by about 4" - the spray pattern generally produces a thinner coat near the outside edges of the fan pattern and is heaviest the nearer the center you get. Donít get too caught up on exactly how much to overlap. Usually a little more is better than not enough, especially in the final coat Ė but watch for those runs. The idea is to lay down a wet coat of paint of uniform thickness. If you overlap too much, you will effectively increase the thickness of the coat and this will result in runs on vertical surfaces. If you don't overlap enough, you will have dry areas between the lines and the finish will not "flow" and produce a consistent sheen.
Practice your spray patterns with a reflecting light behind a horizontal surface and you will be able to see these "dry" areas as you are spraying. As you practice on a horizontal surface, try and learn just how much you need to overlap in order to get a consistent wet coat. Start with a small overlap that produces a relatively dry area between the lines of spray and then overlap more until you have gotten a consistent wet coat. Just like brushing, you need to work to the wet edge, meaning you want to overlap your spray against the preceding line while that line is still wet. It doesnít take very many seconds for latex paints to start to dry on the surface. This practice routine is meant to let you see how little paint you need to put down to get a satisfactory result.
Practice spraying vertical surfaces lets you see how much you can put down before you develop runs. Try and develop a consistent speed and distance that the spray tip is from the surface you are spraying. You'll find the optimum distance from the surface that will give a desired wet coat. Too far away will cause the atomized (micro droplets) of paint to separate and begin to dry on its way to the surface.
Okay, so itís not very practical to practice with an airless sprayer, what with all the cleanup required, not to mention the expense of the paint, considering the high volume that airless sprayers shoot per minute. Use the primer and undercoats to practice your speed, distance and workflow. The workflow is the order in which you move through the project Ė where you begin, how you progress, and where you finish. Before you even think about spraying a project, you need to consider your workflow in order to achieve satisfactory results (minimize runs and over spray that might settle on the already drying surfaces). Make adjustments so that you will be able to put down a good, solid final coat and be done. Trigger control becomes increasingly important if you have to spray inside corners as in cabinet interiors. Basic to spraying anything is that you must start off the surface you want to finish and continue spraying past the end of the surface you are working on. Spraying interior cabinets, especially when the backs are installed, is a real challenge. You donít have the luxury of being able to start and stop the project. Itís real easy to get trapped inside a cabinet, so you will have to develop a technique that allows you to cover the interior surfaces without runs at every corner. I find that trigger control and distance is the key to finishing interior spaces. Learn how to draw your hand and gun out of the box and maintain proper angle and distance from the surfaces being coated. If you get a run (happens even to the most experienced), you will just have to wait for it to dry and sand it back.
Sanding is mandatory after the primer coat(s) if you really want to produce a nice-to-the-touch, smooth finish. I will often sand again just before the final coat. I strictly use sanding sponges for paint primers and between top coats as theyíre fast and sponges are easy on the hands, last longer, and get into the contours very well. Remember, though, that if the wood was not sanded smooth to begin with, no paint or sanding between coats will overcome poor wood preparation. After the primer is applied and has dried, I apply a similar color of paintable latex caulk at all the seams (voids between panels on cabinets and between moldings and walls in the case of wall trim) and in the nail holes if they were not filled beforehand. I found that applying a good painted surface was actually more difficult than applying a standard stain and clear finish to wood. Semi-gloss or satin clear coat finishes will usually hide minor blemishes in the surface but not so with paint Ė a slight bump, dip or nib in paint will be easily seen. I charge more for a painted surface than I do for a normal stain grade finish, but then Iím not set up to routinely spray opaque finishes on any kind of large scale.
With trim work, you should be able to spray the whole width of the trim with a single pass. You will have to take some care when you change directions 90 degrees, and have to overlap back onto the already wet surface.
I use an airledd for spraying trim and doors (in the shop). For painting with SW enamel, I usually add 2-4 cups thinner/gallon paint. For painting trim, I would start with a 213 tip, holding the gun 8-12" away and spray from both sides to get all edges.
The only thing I might add is to use a 4/10 reversible tip, available from Graco. Set your pressure at 3/4 and do multiple light coats without allowing flash time. Have good lighting so you can see how wet you are getting. You can also use a flow enhancer so you don't have to spray so heavy to get a smooth surface. Slowing the flash sounds like a recipe for runs and sags, but if you stop at the point where you still have some orange peel, it will flow out smooth on its own. Do some test panels to get the technique right, and also to check your overlap, as an airless overlaps differently than a conventional sprayer. I use an airless as my main topcoat gun in a high-volume, high-end architectural millwork company and have no plans to go back to conventional.
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