By Dean Bielanowski
Over the years of spray painting and spray finishing many woodworking projects, and a few metalworking ones too, plus experience with model painting with airbrushes, I have picked up a few tricks to almost guarantee a successful result.
I never say anything is guaranteed when it comes to spray painting as there are so many variables involved from air pressure to the thickness and type of paint, to the spray gun type and even the temperature and environmental conditions at the time of painting, but here are some tips that will get you started on the right track. I am not claiming these to work with every situation, but they have worked for me very well in the past! These tips relate to the use of a standard siphon type spray gun for larger projects but many can be applied for spraying with almost all types of spray equipment.
- To begin with, ensure you are using appropriate equipment. For standard spray guns, you require an air compressor that can deliver the required air flow for the spray gun being used. This is normally measured in CFM (cubic feet per minute). Also, your spray gun needs to be kept in top condition, cleaned after each use and checked before each use for free air flow and no blockages in air or paint siphon tubes etc.
- Each spray gun is different. You should read any documents that came with the equipment as these often recommend specific air pressures for the equipment and these are best adhered to for successful results. Nozzle size will have an impact on air pressure required. A 2mm nozzle works better at higher pressures around 50 PSI whereas a 1mm nozzle at 50 PSI will spray paint too forcefully and result in an uneven finish or an “orange peel” effect. Typically, larger spray nozzles require higher pressure. Ensure your compressor or air source has a pressure regulator that you can dial down the supply pressure with. To set air pressure using a regulator, air must be flowing through the hose. So attach an air blow gun, squeeze the trigger and allow air to flow. Now turn your regulator’s pressure setting knob until pressure drops to zero and no air is coming from the air gun. Now with the air gun trigger squeezed again, turn the knob in the opposite direction until air flows and keep turning until the required pressure is showing on the regulator’s gauge. Now you have correct supply pressure.
- Paint mix is critical to a good spray job. Most tins of paint will say to thin the mixture down 5-10% for spraying. This has never worked for me, and thinning by this small amount most often always results in a mix that is still far too thick for spraying. I thin at least 25%, or a 1:4 ratio of thinner to paint. Sometimes more depending on the job. If paint is too thick it will not flow easily through a paint gun and your first coat will be disastrous and require sanding back. You want to achieve the consistency of milk out of the fridge or something near that. This is the type of thing you have to learn as you go. You can buy little tools that help measure a paint’s consistency, but learning the right mix by eye is your best option. A quality finish whether by brush or spray gun usually involves several thin coats of paint as opposed to one thick coat. This is the same principle for spray painting. You need a thinner mix of paint to spray properly so you wont get full coverage in one application anyway, and several thinner coats will ultimately adhere better and provide a more durable finish. Practice achieving the right paint mix by spraying on scrap material, adjusting your spray gun’s air and paint supply knobs (if they have them) as you go to learn what effect each has on the end result. Note also that if spraying varnishes, lacquers or urethane type finishes, these may also have different mix requirements, but again I have found that you do need to thin these a little more than what is recommended on the can too.
- One benefit of spray painting is that, because the coat you apply is usually thinner than what you apply with a brush, and because of a larger amount of paint thinner in the mix (which evaporates as it dries), the time between re-coats is reduced. Typically most paint manufacturers recommend 2-4 hours between recoating with water-based acrylic paints and up to 24 hours for oil-based enamel paints. If you are spraying these paints in thinner coats, you might only need to wait 30 minutes for acrylics and 3-4 hours for enamels if air dried in warm environments with not too much humidity. Again, each paint brand will be different in this regard and dry times vary greatly depending on where in the world you live.
- When applying paint don’t pause over a particular spot just to try and get more coverage; keep the gun moving at an even and consistent pace. Pausing or stopping movement over an area will likely quickly cause excessive paint spray build-up in that area leading to unsightly paint or finish runs which need to be sanded back later. If, for some reason the paint didn’t spray onto a small section during that pass, leave it and cover it on the next pass. Also, when you first squeeze the trigger to start painting, do this away from the material so that first surge of paint exiting the gun (usually under a slightly higher pressure too) does not hit the material causing an inconsistent result. Always try to keep the spray gun running parallel to the face of the material being painted to ensure an even paint coverage.
- Be sure to wear appropriate safety gear for eyes, lungs and body. Glasses or goggles, a paint approved respirator or mask and some overalls will keep you clean and healthy. Also consider overspray and protect items you do not want paint or other finishes resting on.
- Clean up after use… Primarily this means your spray gun. Never leave paint or other finishes sitting in a spray gun for any length of time. Once you are done spraying, and even if you are not coming back for 30 minutes or so, you should empty the spray gun container, add thinner or cleaner, spray that through the gun, then disassemble the nozzle and any needles and thoroughly clean them with appropriate thinner or cleaner for the paint or finish that you have used. It is a bit of a hassle and certainly more time consuming than cleaning brushes, but next time you go to use your spray gun (even if it is for the next coat of finish in the next hour) your gun will be ready and will perform exactly as it should, without any blocked passages, or nozzles that will affect its performance.
- Practice makes perfect. Don’t be afraid to experiment and learn. It may also sound very technical to begin with, but once you go through the motions of setting up, spraying and cleaning your equipment, you learn pretty quick and will realise the great benefits of spray finishing. You can say goodbye to unsightly brush marks and say hello to high quality paint and finish results that will be the envy of everyone who sees your work!
So, in summary, the most important things to consider are:
- Know your equipment and its limitations (if any)
- Get your paint/finish thickness to the consistency your spray equipment will agree with
- Protect yourself first and foremost
- Clean your gear… thoroughly!
Follow these tips and you will be well on your way to mastering the art of spray finishing and spray painting. It is a whole new and exciting world!
