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Intro | Getting Ready | Let's Paint! | About Paints | Application Charts Latex & Alkyd
Paints | Drying, Curing, Hiding
The porosity of paints varies with their pigment-to-resin ratio. Glossy paints have less pigment than flat paints and are less porous. Moisture (vapour) absorption is affected by the resin used and some paints, such as alkyd semi-gloss are effective vapour barriers. Flat latex paint has a high pigment-to-resin ratio, making it very porous, and should never be used as a primer under eggshell paint. When eggshell is applied over flat latex, some of the resin is absorbed by the flat paint, causing an inconsistent sheen in the eggshell. If rolled, the top of the stipple will be glossier than the bottom of the stipple, creating a mottled look. Acrylic latex paints maintain better adhesion when wet, than most other latex paints, which often peel or wash off when wet - even if they adhered well in dry conditions. Alkyd paints tend to yellow with age, more quickly in the absence of light, and in the presence of fumes from epoxy products or from ammonia in cleaning products. Repainting will cover yellowed alkyd. Some alkyd paints contain large amounts of linseed oil and will wrinkle in low temperatures. These types of paint require 2 days drying time at approximately 25ºC before applying a second coat, or wrinkling may occur. Cool temperatures and high humidity will lengthen the drying time required. Fast drying paints applied over an insufficiently dried primer will also wrinkle. HIGH TEMPERATURES: Latex gloss and semi-gloss are affected by high temperatures, and may become sticky in warm, humid weather - and for this reason, are not a good choice for exterior doors or opening windows. The maximum service temperatures for paints are: The viscosity (body) of paints are affected by temperature, and paint will become quite thick in cold weather, and very thin in hot weather. Paint left exposed, or left in vehicles in hot weather, will have to be brought back to 25ºC before using. Cold paint doesn't flow well and tends to sag because of going on too thick. Hot paint has poor coverage because it goes on too thin. LATEX vs. ALKYD PAINT: It takes time for latex or alkyd paint to cure. Latex paints are usually dry to touch in 1 hour and ready for a second coat after 4 hours, but it might take three weeks before all the solvents come out of the finish. The coating will not provide its best water resistance until after the sovents have evaporated. Alkyd paints usually cure as soon as they've dried... about 16 hours after application, however full hardness for an alkyd paint is not achieved until 3-4 weeks! Alkalinity from damp concrete attacks the resin in alkyd paint, making it soft and sticky. (Curing: A chemical reaction dependent upon temperature.) HIDING: The "hiding" ability of paint is determined by the thickness of the application and the amount of pigment in the paint. When a major colour change is in the works, there is a popular belief that the surface should be primed before being repainted - but in fact, white base and some medium-tint bases have better hiding ability that primers! For better hiding of clear-base deep colours, use a tinted primer or if applying latex on drywall, use the finish paint as a primer. It is not advisable to use more than 4 ounces of tint per gallon of paint. Latex eggshell or semi-gloss make good primers for alkyd eggshell or semigloss on drywall. Matching the colour will minimize the number of coats needed for total hiding. Flat, satin or eggshell paints are popular for hiding drywall taped joints - because the lower the sheen, the less visible any flaws will be. But the lower the sheen, the more susceptible the paint will be to burnishing (developing shiny areas) when rubbed or washed. Burnished areas have to be repainted. OLD BUILDINGS: Shadows may appear at framing in older buildings where the insulation is poor or non-existant. The temperature differential attracts dirt to these areas. Repainting will cover these shadows, but they may reappear if the insulation in not improved. LINSEED OIL: Sold in many stores, boiled linseed oil is used as a clear finish, however it is exceedingly slow drying, yellows badly and tends to attract mildew. Alkyd house paints are alkyds mixed with oils to make them more flexible for exterior use. This makes them too soft for porch paint and if used indoors, will tend to yellow more than regular alkyd paints. Paint will normally "chalk" with age, as the resin breaks down in sunlight, leaving the loose pigment (chalk) on the surface. Surfaces should be cleaned and painted before chalking becomes excessive. If left too long, the surface will deteriorate and complete removal, priming and repainting will be required. Dirt will promote the growth of mildew, and all exterior surfaces should be washed periodically, especially in humid areas. Dark colours absorb much more heat than light colours. Siding can warp, and sap may be drawn out, causing the paint to peel. Black paint causes cracking and cupping of wood in direct sunlight.
To determine if a painted surface is latex or alkyd, rub the paint with nailpolish remover. Latex paint will come off, whereas alkyd paint will not be affected! When looking at low-sheen colour chips, wet the chips with water to see what the colour will look like in an eggshell or semi-gloss! "Gloss" is measured at a shallow angle
(almost straight on), at a short distance from a surface. To estimate the number of coats of paint on a surface, measure the thickness of a paint chip with a micrometer. Divide the thickness by 1.5 to determine the number of coats! The solvents in alkyd primer will raise the nap of the drywall paper, requiring extra sanding before re-coating. Latex sealers do not raise the paper nap and can be top-coated with alkyd or latex. Reflection of light is a function of colour only. Neither gloss or sheen have anything to do with it!
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