CoatQuotaHow much paint a room takes, coat by coat.

Exterior paint for 140 ft of T1-11 siding at 10 ft high, 2 coats

Exterior paint
10.73gallons
1,148 ft² of T1-11 plywood siding in 2 coats — 3 × 5 gal buckets
Exterior paint by coat5.74 galfirst coat4.99 galeach later coat15% further once the surface is sealed — 0.75 gal saved10.73gallons total200 ft²/galtexture, not brand
Paint10.73gal
What to buybuckets pay from about 3.6 gal up3 × 5 gal
PrimerT1-11 plywood siding is porous — primer is a separate coat, not an option6.75gal
Wall area140 ft perimeter × 10 ft, less 14 windows and 2 doors1,148ft²
Spread raterough face plus deep grooves — thirsty200ft²/gal
First coat5.74gal
Each later coat15% further on a sealed surface4.99gal
Doubling the first coat would say0.75 gal more than needed11.48gal
Perimeter
140 ft
Wall height
10 ft
Siding
T1-11 siding
Coats
2
Windows
14

Texture decides the spread rate, not the brand

T1-11 siding takes about 200 ft² per gallon. The same house in smooth fiber cement would take noticeably less, and in brick noticeably more — the mortar joints drink more than the brick does.

140 ft of perimeter at 10 ft high, less 14 windows and two doors, is 1,148 ft². At 2 coat{lop|s} that is 10.73 gallons.

The second coat is cheaper

Once the surface is sealed, paint spreads about 15% further. Doubling the first coat would say 11.48 gal — 0.75 gal more than you need, sitting in the garage until it separates.

Two weather limits matter more than the gallon count

Surface temperature has to stay above about 10 °C for the whole cure, not just while you are painting. A warm afternoon followed by a cold night gives a film that never properly coalesces, and it comes off in sheets the following spring.

Dew point matters as much as rain. Early-morning painting on a clear autumn day fails constantly for exactly this reason — the siding is wet with dew that you cannot see.

Wash first, and wait

Paint over chalky or dirty siding fails at the interface however good the paint is. Pressure-washed wood needs two or three dry days; a moisture meter reading under 15% is the honest test, not "it looks dry".

Back-roll anything you spray

Spray lays paint on the surface. Back-rolling works it into the grain and the grooves. On T1-11 siding that is the difference between six years and twelve.

Notes

  • **Surface texture, not brand, decides the spread rate.** T1-11 plywood siding takes about 200 ft² per gallon — rough face plus deep grooves — thirsty. The same house in smooth fibre cement would take 3.28 gal per coat against 5.74 here.
  • **Two weather limits matter more than the gallon count.** Surface temperature must stay above about 10 °C for the whole cure, not just while you are painting — a warm afternoon followed by a cold night gives a film that never coalesces. And the surface has to be dry: dew point matters as much as rain, which is why early-morning painting on a clear autumn day so often fails.
  • T1-11 plywood siding needs primer as a separate coat — 6.75 gal. On brick and stucco a masonry primer also seals the alkalinity that otherwise burns through the topcoat as patchy discolouration months later.
  • Wash it first, and let it dry properly. Paint over chalky or dirty siding fails at the interface no matter how good the paint is, and pressure-washed wood needs two or three dry days before it is ready — a moisture meter reading under 15% is the honest test.
  • Back-roll anything sprayed. Spray alone lays the paint on the surface; back-rolling works it into the grain and the grooves, and on T1-11 plywood siding that is the difference between six years and twelve.
  • Buy it all at once and box it. Exterior walls are seen in full sun from a distance, which is the least forgiving lighting there is for a batch difference.

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