Mats Bengtsson Home Improvement and Do It Yourself

In depth description of fine tuning the spray gun settings

Mats Bengtsson mib over the years

In depth description of how to fine tune the spray gun settings

This description starts after you have made and found the initial settings that work for your spray gun, as described in Adjusting the paint and air flow through the spray gun and the pressure cup. This means that you already have:

  • A suitable needle size, fitting the paint
  • A suitable maximum paint flow, allowing you to work on open surfaces with a suitable spraying speed
  • A suitable maximum paint fan with, allowing you to work on open surfaces with a suitable spraying speed and covered area
  • A fan shape that indicates no problem with settings or with spray gun/needle/nozzle in terms of dirt or clogging.

If all surfaces you where to paint where large open surfaces where you could easily move the gun, and where neither air flow or paint flow would hit adjacent corners, your setting was done. But even if the mentioned ideal situation is what you should strive for (more or less paint your work before you assemble it), reality sometimes is that this can not be done.

Situations where adjustments are needed to match the shape of the material

You need to make adjustments of the flow when the material to be painted is narrow, or if it contains a shape which will block or deviate the air flow:

  • The most common exception to the large open area is an edge. On a door or cabinet side, you will have an edge which most likely is quite narrow compared to the door or cabinet side itself.
  • For a cabinet, even if the shelves and doors are not yet mounted, a bottom and a top is likely to be mounted before you spray the cabinet, creating inside edges and corners.
  • It would be even worse if you also had either the cabinet backside already mounted, or a number of frames or beams.

As explained in the litterature and videos that exist on spray painting, you start spraying in the least visible areas (overspray onto those areas will thus not matter that much for the final appearance). And you also want to take all corners and edges early, so that you do not have to find a way to paint them with a hard to achieve move of the gun from one open surface area to another one. Such a move would probably involve a 90 degree angle to the paint direction you were just following.

  • The most obvious adjustment of the spray gun you should get used to is directing the fan in horisontal or vertical directions, or something in between. It is very simple to do, just turn the forward fan ring on the spray gun, so the fans wide part faces in the direction most suitable for you. That is a setting you are likely to want to change many times. It does not affect the other settings, so I will not discuss it further here, but it is an essential thing to do when needed, and much better than angling the paint gun itself..

For corners lower the flow through the paint gun

A problem that you are likely to encounter when doing corners is the tendency for paint and air to bounce. Also, when you later add the surface paint on the flat area close to the corner, a tendency is to get too much paint in the corners, causing runs or uneven surfaces. To prevent these problems as much as possible, you should have a much smaller flow through the paint gun. Thus you need to do an adjustment. The adjustment made will affect a lot of things, so a follow up with other adjustments are also needed:

First flow adjustment to make is the fan width itself

Starting with the fan width, for a 90 degree inside edge between a bottom and a side, you probably want to reduce the fan width you would normally be using, maybe all the way down to a minimum round pattern. On my spray gun, the adjustment between a suitable fan width of around 15 centimeter at normal spraying distance, and the minimum round fan, is achieved with a turn of the fan width knob of around 3/4 of a full turn. When doing that adjustment, a number of other things things are altered:

  • The paint is spread over a smaller area. Which means as long as you do not adjust the paint flow, you will get more paint coverage in less time than before. This is the opposite of the defined goal with the change, so it has to be countered with further adjustments, or you are in danger of getting runs.
  • The air is spread over a smaller area, increasing the amount of air hitting the edge where you want a decreased flow. This is also opposite the defined goal with the change. However, until you have reduced the paint flow itself, it is to a certain degree also needed, because the increased amount of flow in the same area need to remain atomized.

Second flow adjustment to make is the paint flow

  • After adjusting the fan width, you now as a consequence should lower the paint flow through the spray gun. Not by changing the pressure in the pressure tank, but by adjusting how much paint you can get out of the spray gun with the trigger fully pulled. This is done by turning the knob behind the spray gun, until the needle does not open more than the amount of paint you want to get out. This adjustment actually does two things:
    • It first reduces the amount of paint until the paint flow onto the covered area is no longer higher than before adjusting the fan width. (This is since the decreased fan increased the amount of paint flowing into a specific area, since the amount of paint coming out of the spray gun remain the same until the needle movement is reduced ).
    • After that it reduces the amount of paint actually hitting the piece, which was the original goal with the adjustment. To judge this, what you need is some kind of test paper or wood, onto which you can test how much paint goes onto it, by test spraying. The more adjustments you need to make during a spray session, the more material you need to have available for these kind of tests. Even worse, there is no other way than test and experience to judge what flow of paint you want, or how much you need to turn the knob adjusting the amount of needle you can pull back with a full trigger. Thus, best suggestion besides always testing after changes, is to count number of turns, and learn to make adjustments counting number of turns. You will benefit by knowing how many full turns there is between the maximum point (where the paint flow knob start pressing on the fully pulled trigger) until the paint flow is off (trigger can not be pressed passed the "air only" pressure point). This will after a while give you good knowledge both on how much adjustment is suitable, as well as knowing how much to reverse the knob to be back at start settings again when done with the part needing reduced flow.
  • After now having the paint flow as well as the fan adjusted to a suitable setting, comes the time to adjust the air flow. The needed air flow will be lower due to the reduced amount of paint. However, the air flow is depending on the gun, the nozzle size, and the type of paint as well. Thus, the reduction suitable is likely to not be so much as could have been expected if it would be corresponding to the reduced amount of paint. On my gun, I have at the gun an air flow reduction and a digital air pressure reading. From that, I know a reduction of the pressure of 10% can be made, but to reduce it 20% may cause splash patterns to appear in the test spray picture unless I have reduced paint flow a lot. The reduction of 10% may not sound much, but then remember that just as for the paint, the reduction of the fan has reduced the amount of air needed, so there is first a reduction needed to do to get back down to the original air pressure, and then comes the 10% reduction of air pressure.

Fine tuning spray gun settings further based on the results

When the paint has dried, or almost dried, you can study your results to learn what to adjust further:

  • If you do have runs on your edges, then you have not lowered the flow enough. So whatever the amount of turns you used for lowering the paint flow need to be increased. With that, as defined above, the air setting also need to be adjusted in accordance. However, how much has like described above be based on the paint flow.
  • If you have poor coverage on your edges, there might be two different things to alter. One thing could be increasing the air flow. But one I think should be always considered first is the paint technique:
    • For all areas you paint, you paint best by painting with overlaps (50% overlap or more). This also holds true for the edges and for the ends of open surfaces. Thus:
      1. When painting the edge, do two runs with the paint gun. The first one with the paint gun halfway outside the edge. The second one overlapping the first flow, most likely meaning spraying outside the other side of the edge. This is more similar to your normal paint technique than trying to paint the edge with one single run, aiming to have the edge in the middle of the fan.
      2. When painting the start or the end of an open surface, just like for an edge, start with the paint gun outside the surface. Thus the first overlapping run with the paint gun gives as much paint onto the start of the open surface as next will give further into the open surface. In the same way, when reaching the end, do one more run, with the center of the fan outside the material you are painting, to assure the overlap and the consistency in amount of paint.


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