Advantages Of Oil VS. Latex
Latex finishes: Resist fading, do not chalk, easy clean up, low voc, flexible, breathable (allows moisture to pass through), fast dry, requires no chemical solvents.
Oil finish: flows out during application, protects metal, washes well, resist smearing and marking, penetrates into chalky surfaces, After drying it will not become stuck to rubber weather stripping from contact, Strong initial bond to most surfaces.
New Wood: Any new unprimed wood should first be coated with a suitable primer before the finish paint is applied. Oil\Alkyd was once considered a better choice than latex for wood primer. One reason that the oil may seem to be the better choice is that many painters feel that it's resins will penetrate into the wood surface better than the fast drying latex. That is probably true to a certain extent, but where there is usually some moisture content in the exterior wood, the oil may not be able penetrate as much as it anticipated. Where latex paint will mix with water, some moisture content in the wood will not resist the latex primer. Another property of latex is that allows moisture to penetrate (Breaths) much more so than oil .The breath-ability of latex may help prevent paint from pealing when the moisture is drawn out through the siding by the difference in interior and exterior air conditions. Another benefit of latex is its flexibility, as the siding will expand and contract with temperature and humidity changes.
An advantage of using oil based primer for wood siding is it also makes a somewhat good metal primer that will seal nail heads protecting them from rust more so than the latex. Also the oil-based /Alkyd does not raise wood grain.
We would not recommend using a latex primer to be coated with an oil finish in most exterior cases due to the difference in flexibility, but an oil/alkyd primer under latex finish paint is a good method. However sometimes it is necessary to use an alkyd based stain sealer over a latex ceiling or wall paint
Whatever type of primer you chose, check the manufacturers label
to be sure it is suitable for the materials you will be using it on,
there may be conditions such as tannin bleed associated with certain
kinds of wood. We have been quite pleased with some of our more
recent experiments with the latex wood primer. Although latex primer
has increased in popularity in recent years, many are still more
confident in the oil/alkyd primer.
For more information see What's in a can of Paint
For the most part previously painted wood will not need to be primed, however spot priming all bare wood and rusted nail heads is important. If you will be using oil finish paint, then you should use oil based primer. If you will be using latex finish paint then you can use latex or oil primer. If there are a lot of rusted nail heads showing through, then you may choose to use the oil primer as it will usually seal metal better than latex.
Wood Trim, Wood Siding: Latex house paints have been out selling oil based house paints for years. For the most part the latex is the more practical choice, however there are cases where using oil will be preferred. Houses previously painted with oil based house paint may have developed a chalky surface. If this chalky surface were not washed prior to painting, latex paint would not develop as good a bond to the surface as the oil based paint. That’s because the oil-based paint can penetrate through the chalk better than the latex can.
Chalking paint was a way of having a self cleaning building. The top surface of the paint gradually turns to a fine powder, which gets washed away with the rain leaving a cleaner outer appearance. But the chalky effect is not as desirable today as it use to be and most paint available is anti chalking, even so most exterior oil based paints will eventually chalk to a certain degree.
Oil /Alkyd is often the better choice for exterior doors. This is because often the latex paint, even completely dry will form a bond to certain rubber-like materials in which the doors weather stripping is made of. This will lead to pealing paint wherever the weather strip contacts the door.
Window sash is often easier to paint with oil/alkyd, than with latex. If widows are being re-glazed with oil based glazing compound (Window Putty), they will need to be painted or at least primed with oil paint.
Vinyl Siding: Latex is generally preferred for vinyl siding. With a good quality latex paint, no primer will be necessary, but a latex bonding primer is a good idea on glossy siding.
Vinyl has a higher rate of thermal expansion than most building materials, in other words it will expand and contract further with temperature change. For that reason it is advised that you do not paint vinyl siding with a color that is darker than its original color, as dark colors draw solar heat. Individual siding manufacturers often have there own painting recommendations. Some manufacturers recommend not to paint it
Aluminum Siding After washing, latex finish paint gives nice long lasting results no primer is usually required .Bare spots can be spot primed prior to painting with the finish paint.
Primer: Unprimed hardboard may produce some bleed through with latex primer, its usually recommended to use an oil base /alkyd primer for these products. However with the wide array of materials available, it it wise to check with the manufacturer, as with some of the newer cementasious siding products it is recommended that they are primed with latex.
Finish Paint: Latex Paint will bond to latex or oil primer. Oil finish paint will bond to the latex or oil primer, but may not always work out so well over the latex due to the higher flexibility of the latex
MetalMost new metal installations come pre-finished, or primed .Any bare un-oxidized should be primed with the proper type metal primer. Ferrous metals (steel, iron) can usually be primed with a standard gray or white metal primer Oil Based.
Galvanized metal should be primed with a product designed for that purpose Galvanized Metal Primer. Usually White.
Aluminum can be primed with the galvanized metal primer, or the standard gray primer. Latex also bonds to aluminum and can be used for a primer under latex finish paint.
On slightly rusted surfaces use gray metal primer. Heavily rusted surfaces use a red (Iron Oxide) metal primer.
In general; (but not a rule of thumb) the more rusted the
surface, the darker the recommended primer will be. Other commercial
grade primers are also available for certain exterior use.
Finish paint Once properly primed, most metal interior metal surfaces can be painted with almost any quality paint. However metal door and heat registers usually fair better with an oil base/alkyd.
Gloss Metal Enamels provide the most durability, but can sometimes be more difficult to work with, and the high gloss may be over bearing. Semi gloss or eggshell gloss trim paint is a good choice for steel doors.
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