Spraying with HVLP

Overview

HVLP (high volume, low pressure) guns atomize finish with a large volume of air at low pressure. Fuji, a turbine maker, defines HVLP as spraying below 10 psi, against 20 to 90 psi for conventional compressor systems, and says the gentler spray means less overspray and bounce-back. A turbine pushes air through the hose from a few fan stages; a compressor-fed HVLP gun uses a regulator. In a small shop, it is the standard way to get a flat, fast-drying, nib-free film on furniture and cabinets, and it suits the fast-flashing finishes that are hard to brush: Nitrocellulose lacquer, Pre-catalyzed and post-catalyzed lacquer, Water-based lacquer, Shellac and waterborne topcoats.

When to use

• Cabinets, case goods, tops and chairs where you want a uniform film in few coats; see Tabletop finish schedules.
• You can set up a dust-controlled, ventilated spray area with a respirator.
• Not for a one-off small box (a rattle can may serve; see Spraying aerosol finishes) and not for very heavy coatings that need undiluted pumping, which suit Airless and air-assisted airless spraying.

Materials and tools

• Turbine or compressor HVLP system. Fuji says its thinner finishes (stain, lacquer, varnish, urethane) can be sprayed straight from the tin; thicker ones such as latex or tinted lacquer need thinning, and a 4 or 5 stage turbine helps with them.
• Gun with pressure-assist cup so it sprays at any angle (Fuji). Bottom-feed quart cups suit large work; a 400 cc gravity cup is handier inside cabinets.
• Air cap sets. Fuji ships 1.3 mm; it says 1.0 or 1.3 mm suits thin clear finishes, with larger sets for heavier materials.
• A #4 Ford viscosity cup, the maker's thinner, kraft paper, scrap board, respirator, gloves and eye protection.

Steps

1. Sand and clean; see Sanding schedule. Spray shows every scratch.
2. Strain the finish, then time it through a #4 Ford cup. Fuji's guide: about 8 to 17 seconds suits a 1.0 mm cap, 17 to 25 seconds a 1.3 mm, 25 to 37 seconds a 1.5 mm, 37 to 50 seconds a 1.8 mm, and more than 50 seconds a 2.0 or 2.5 mm. Thin only as much as needed, using the solvent the finish maker specifies; see Thinning and viscosity.
3. Test on scrap. Open the fluid knob and adjust fan together; use a wide fan on panels and a round one on spindles.
4. Hold the gun square to the surface, 6 to 8 in away (Fuji). Start the pass off the work, pull the trigger, move steadily, and release only after you pass the far edge.
5. Overlap each pass by at least half. Fuji's non-bleed guns have a two-stage trigger: the first pull releases air, further pull releases fluid.
6. Spray edges and hidden faces first, the top last. Aim for a wet, even coat, not heavy.
7. Depressurize the cup before opening it, to avoid a droplet pop (Fuji). Clean the passages straight away.
8. Recoat per the finish maker; see Build coats and sanding between.

Common problems

• Orange peel, runs, sags and dry spray: see Troubleshooting finish defects; usually viscosity, distance, or speed.
• Poor atomization: too thick for the cap; thin slightly or use a bigger cap.
• Blush: humid air with fast-evaporating lacquer; see Curing conditions and time-to-use.
• Fine-finishing a very large run: aerosol-style strips drying before the next pass is a problem for cans, not guns (Fine Woodworking).

Safety and environment

Spray mist and solvent vapour are hazards. Fuji's manual says to read the safety data sheet, wear gloves and eye protection and always wear a respirator. Keep ignition sources away from flammable finishes, ventilate and ground the turbine as directed. See Oily rag safety for rag disposal.

Related

• Spraying aerosol finishes, Airless and air-assisted airless spraying: other spray routes.
• Sheen control and matching: sprayed satin versus rubbed satin.
• Nitrocellulose lacquer, Pre-catalyzed and post-catalyzed lacquer, Water-based lacquer, Shellac: spray finishes.