How to Spray and Back-Roll Exterior Siding

Why this matters

A sprayed first coat on rough-sawn cedar looks complete from the driveway and is full of pinholes from three feet away. The spray lands on the high fibers and bridges the checks and pores under them, and those bridges are where water gets behind the film in the first winter. Back-rolling, a roller following the gun while the paint is still wet, is what drives that coat into the wood. Done with the roller too far behind, or stopped mid-board, it leaves drag marks and lap lines. This card owns the two-person sequence: where each person stands, how far apart, and where a course is allowed to start and stop.

What the roller is actually doing

On a smooth, sound, previously painted board a sprayed film is fine as laid. On a porous or textured surface it is not:

• It sits on the peaks. Spray deposits on the first thing it meets: the raised fiber of rough-sawn wood, the top of stucco texture, the face of brick rather than its pores.
• It bridges openings. Checks in old cedar, nail holes, the pores in masonry get a skin of paint over air. The skin looks continuous and is not bonded to the bottom of the opening.
• It traps air. Bubbles in spray over a porous surface burst into pinholes as it dries.

A loaded roller, pushed through the wet film, forces paint into the pores and checks, levels the film between peaks and valleys, and pops the bubbles before they skin. The first coat on a porous or rough substrate is the one that needs it, because it is the coat that bonds and seals. A second coat over a sealed first coat is often fine sprayed alone; where the data sheet says which coats it expects back-rolled, follow it.

Before you start

Safety first, because the gun is the hazard. An airless pump runs the fluid at pressures that will inject paint through skin, and an injection injury looks like a pinprick and needs a surgeon the same day, not a bandage. Tip guard on, trigger lock on whenever the gun is not pointed at the wall, and never a finger or palm across the tip. Relieve pressure per the pump maker's procedure (pump off, prime valve open, gun triggered into a grounded pail) before clearing a clog, changing a tip or handing the gun to someone. The tip and pressure come from 'How to Pick and Test an Airless Tip for the Coating', within the coating maker's tip range and the pump's maximum tip rating.

Ladders, poles and power lines. Look up before the first ladder goes up. Ladders, extension poles and the spray wand stay at least 10 feet from overhead lines up to 50 kV, the clearance OSHA sets for unqualified workers in 1910.333(c)(3), and the service drop to the house is an overhead line. Work classed as construction falls under 29 CFR 1926 Subpart K and the ladder rules in 1926 Subpart X. Ladder setup belongs to 'Ladder and Scaffold Setup for Painters'.

Lead. In a pre-1978 house, presume lead until an EPA-recognized test kit or a certified renovator's sample says otherwise. Spraying a new coat is not disturbance; the scraping and sanding that came before it is, and that is 'How to Scrape and Feather Sand Peeling Trim on a Pre-1978 House'.

Overspray. Windows, doors, fixtures, the walk, the cars and the neighbor's side are masked and covered before the pump starts, per 'Spray Containment and Overspray Control'.

The surface and the weather. Washed, dry, scraped, primed where the primer map says so, caulked where it should be and left open where it should not (see related: The Gaps a Painter Must Never Caulk). Surface and air temperature, dew point and the recoat window come from the product data sheet; see 'How to Read a Product Data Sheet for Its Weather Limits'. Do not start a wall the sun will reach before you finish it.

Gear. Pump and gun set up and tested on cardboard. A roller with a nap chosen for the texture, typically 3/4 inch to 1-1/4 inch on rough-sawn wood and stucco, per 'Spray Vs Backroll Vs Brush Masonry Coverage: Decision Matrix'. Roller pre-loaded before the first pass, so a dry cover does not pull paint off the wall. A brush for butt edges and corners. A wet film gauge.

Plan the wall into bays

Before anyone sprays, the lead walks the elevation and breaks it into bays: the run of siding between two natural stops. A natural stop is a corner board, a window or door casing, a vertical trim board, or a butt joint where two boards meet end to end. Each course of siding is painted from one stop to the next in one wet operation, sprayed and rolled, and the crew only ever pauses at a stop.

The reason is the lap mark. Paint that has started to set will not blend with fresh paint laid over its edge; the overlap dries a different sheen and a different color depth. A board painted half today and half after lunch carries a visible line where the two halves met.

The sequence

1. Paint the butt edges ahead of the face. The butt is the thick bottom edge of each lap board. A roller on the face does not reach it. As each pair of courses comes up, either brush the butts or give them a spray pass with the gun angled up into the edges. Skipped, it is the most weather-exposed edge on the board left bare or thin, and peeling starts there.
2. Sprayer starts at the top course of the bay. Hold the gun about 12 inches off the face, perpendicular to the wall, start the arm moving before you pull the trigger and release the trigger before the arm stops, so the pass lays an even film to the stop instead of a heavy spot where the arm paused. Move at a steady pace that lays the wet film the data sheet calls for. Overlap each pass about half its width.
3. Sprayer runs two courses ahead, full length of the bay. Two courses, stop to stop, then down to the next two. The distance matters: close enough that the paint the roller meets is still wet, far enough that the two people are not working the same board.
4. Roller follows on the courses just sprayed. Roll along the length of the board, firm pressure, loading paint into the grain and checks. Finish each course with a light stroke the full length from one stop to the other, so the roller texture runs one direction and ends at a stop. Wipe any paint the roller pushed down onto the course below so it does not dry as a ridge.
5. Close the bay before either person leaves it. Both finish at the bottom course and the bay is done stop to stop. Then move to the next bay.
6. Brush the inside corners and the ends at the casings. The roller cannot get into a corner where siding meets a casing or corner board, and the spray there usually sits on top. A brush works it in while it is still wet.
7. Check the film as you go. Read the wet film gauge on a smooth area such as a corner board or a casing, sprayed in the same pass, because the teeth of a gauge read nothing useful on rough-sawn texture. Compare against the data sheet. 'Film Build and Wet Film Thickness Verification' owns the gauge technique and the spread-rate math behind it.

The geometry of the pair

 corner                                     window
 board                                      casing
  |                                            |
  |== course 1   rolled, finished =============|
  |== course 2   rolled, finished =============|
  |== course 3   ROLLER working, paint wet ====|
  |== course 4   ROLLER next ==================|
  |== course 5   SPRAYER working ==============|
  |== course 6   SPRAYER next =================|
  |-- course 7   bare, butt edges next --------|
  |                                            |
 STOP                                        STOP
 each course runs stop to stop in one pass;
 a pause happens only at a corner board, a
 casing or a butt joint, never mid-board

Two courses is a distance; the time gap is what matters, and it shrinks as the wall heats. The cue that the roller is too far behind: it starts dragging, the paint pulls into ropes, or the texture stays rough instead of leveling. Close the gap before the next course.

Worked example: a rough-sawn cedar front elevation

The house. A 1994 two-story with rough-sawn cedar lap siding at a 6 inch exposure, previously painted, washed and scraped two days earlier, spot-primed on bare wood. Built after 1978, so the lead presumption does not apply. Coating: an exterior acrylic latex satin; the data sheet calls for back-rolling the first coat on rough or porous surfaces.

The bay. The first-story front runs 40 feet wide by 8 feet high. A front door and two windows break it into four bays: 9 feet from the left corner board to the first window casing, 8 feet between window and door, 9 feet between door and second window, and the remainder to the right corner. The left bay has 16 courses (8 feet of height at 6 inches of exposure is 96 inches divided by 6, which is 16).

The passes. The tip was tested on cardboard at about a 10 inch fan at 12 inches. Two courses are 12 inches tall. At half overlap, each new pass adds about 5 inches of fresh band, so two face passes lay roughly a 15 inch band, enough to cover the 12 inches of two courses with the edges blending into the bands above and below. Butt edges got one up-angled pass per two courses before the face passes.

The gap that held and the one that did not. At 9 a.m. in shade, two courses ahead was comfortable. The roller kept the film leveled and the checks filled. By 11 a.m. the sun had reached the right-hand bay, and on course 5 the roller began dragging. The lead had the sprayer drop to one course ahead and slowed the pace. The roller stopped dragging on the next course.

The film and the paint used. Say the data sheet calls for 5 mils wet. 1,604 divided by 5 is about 320 square feet per gallon in theory. The front first story was 320 square feet gross; the door at 21 and the two windows at 15 each take out 51, leaving 269 square feet. At theory that is 269 divided by 320, about 0.84 gallon. The crew used 1.25 gallons. Effective spread was 269 divided by 1.25, about 215 square feet per gallon, roughly two-thirds of theory, which is the same as using about one and a half times the theoretical paint (1.25 divided by 0.84 is about 1.49). That is the roller putting paint into the grain and the checks where the spray alone would have bridged them. The estimate for the remaining three elevations was adjusted to the measured rate, not the data sheet figure.

The check that caught something. Walking the left bay with a work light at a low angle after the coat set, the lead found a run of pinholes along the bottom two courses, the last ones rolled before a break. Those had been rolled after the roller cover went dry, so it skated over the film. They were worked in with a brush and the first-coat paint before the second coat went on, not left for it to bridge.

What would change the answer.

• Smooth fiber cement, previously painted and sound. Spray alone is usually fine on the second coat. The first coat is often still back-rolled for adhesion and to level the film, if the data sheet says so.
• Stucco. Roll in more than one direction to reach both sides of the texture, use the heavier nap, and stop at control joints and corners rather than board ends.
• A pre-1978 house. The scraping and sanding before this coat was RRP work once it passed the 20 square foot exterior minor-repair limit, and had to pass its own cleanup and visual verification before the pump came out. The spray and back-roll themselves disturb no old paint.
• Wind. Gusts put the coat on the neighbor's car. Spray stops; the crew brushes and rolls by hand or the day ends.

How to know it is right

• No pinholes or bridged checks visible with a light raked across the dried first coat, especially at the start and end of each course and in the bottom courses of a bay.
• No lap lines mid-board when you stand back at a sharp angle to the wall. Breaks in sheen fall only at casings, corner boards and butt joints.
• Butt edges coated on every course, checked with a mirror or by looking up from the ladder, not from the ground.
• Measured spread rate written down per elevation, and the remaining gallons revised from it.

References

• OSHA 29 CFR 1910.333(c)(3) overhead line clearance; 29 CFR 1926 Subparts K and X for construction work
• Coating manufacturer's product data sheet and airless pump owner's manual
• See related: How to Pick and Test an Airless Tip for the Coating
• See related: Spray Containment and Overspray Control
• See related: Film Build and Wet Film Thickness Verification