Although similar, both serve quite different functions. Primers act as a foundation for your paint to stick to while undercoats create a flat and level base for topcoats. One easy way to remember is if a surface is painted use an undercoat, if it's new, use a primer.
A rule of thumb to stick with is that undercoats can be primers, but primers can never be undercoats. Here is a little information on what makes Dulux Trade primers and undercoats special.
While both primer and undercoat are used to prepare the surface for painting, they serve a different purpose. They are essentially two steps of one whole process. A good rule to keep in mind is – if you're painting a bare surface, use primer. If the base has been primed or previously painted, use undercoat.
Primer is used to create a base for the paint, improve adhesion, and prevent stains and bleeding. Sealer, on the other hand, is used to seal the surface and prevent the topcoat from being absorbed or bleeding through. Undercoat is used to cover imperfections and create a smooth surface for the topcoat to adhere to.
Undercoats are simply a prepping layer without properties to protect the surface or improve adhesion. Professionals would advise using a coat of primer or undercoat before applying your desired colour for the best results. Primer and undercoat cannot be used instead of topcoat paint.
It's essential to use the correct primers and undercoats to allow paint to do its job properly. Undercoat is often required to seal unpainted surfaces or prepare the surface for painting. If you are transitioning from a dark to a light colour, the undercoat will help to obliterate the colour beneath.
If the existing paint is in good condition and you're using the same type of paint (water-based over water-based or oil-based over oil-based), you may not need to undercoat. Lightly sand the surface to promote adhesion and then apply the new paint directly.
Bare wood is not only porous, but because of its grain variations and the possible presence of knots or filler, absorbs at variable rates, and should always be primed.
Next is the undercoat (unless you are using a multi-purpose primer and undercoat): again, one coat may suffice or two might be needed to cover dark colours. Now for the topcoat – the colour you want the walls to be. Two top coats is the norm, but sometimes extra coats are necessary to get the required evenness.
You will want to use 2-3 coats of primer to ensure there is a good bond between the new paint and the wall, and also to cover up any previous colors, especially if they are red, orange, or a strange outdated color. In short, you will typically need 2 coats of primer for most painting projects.
If you are covering a dark colour or a surface that's never been primed or painted, you will need to use an undercoat. It can also provide a better coverage and hiding power than just applying extra top coats.
After your first coat of paint is dry, it's safe to recoat typically after four to six hours. A good rule of thumb is to wait at least three hours to recoat your paint or primer if it's water-based. Waiting 24 hours is best for oil-based paint and primer.
However, primer is an essential step if you want a smooth finish on projects that have darker colors, wooden or uneven surfaces, or on surfaces that are stained or odorous. If you don't use primer, you could face a lot of challenges which will result in an uneven finish or require more coats of paint.
Product Description
A Undercoat with superb hiding power, good cling to sharp edges and excellent levelling properties. It can be applied on interior or exterior wood and metal surfaces.
Learn which undercoat colours enhance the hiding power of which topcoat colours. For example, straight untinted white would be the correct undercoat colour for a bright lemon yellow topcoat colour. Red finish coats are best undercoated with a more robust hiding light grey tinted undercoat.
You can apply it directly to your uPVC door without the need for a primer. Plus, it guarantees to give you 6 years of weather protection, to keep your uPVC door looking great for longer. By using the Dulux Weathershield system on the outside of your home, you're also protecting what's inside.
Once you have prepared the area I would then undercoat, and give it a light sanding with a fine grade paper to get a good bond for the gloss to stick to. This way you will get a first class finish.
You probably don't need a primer. If the current wall is smooth, clean and covered with the same type of paint (both are oil-based for example), you can head straight for the paint. You need to fill holes, spackle and sand*, maybe even cut a new piece of drywall in to the existing wall.
The primer coat doesn't have to be perfect, but it should cover the surface (no bare spots) and it shouldn't be so blotchy that you get drips or visible unevenness.
Covering a dark paint color with a lighter paint color will require multiple coats of new paint. It may make more sense to cover the old color in a primer, which is less expensive than your paint. This way, you will only need 1 to 2 coats of your new paint.
Acrylic Primer Undercoat can be used on a variety of correctly prepared surfaces such as wood, metal and plaster.
All surfaces to be painted should be clean, dry and free from loose and flaking material. Prime bare wood surfaces with Johnstone's Trade Aqua Water Based Undercoat.
If you've got new or ready-primed skirting boards, or they've already been painted before, you can skip this step. But if your skirting boards are wood-finished or haven't been painted previously, you'll need to apply one to two coats of primer to them before painting.
Undercoats are specifically formulated to create a uniform, adhesive surface to apply paint or gloss over. Emulsion is a topcoat. If you use it as an undercoat, it might dry to a smooth finish but it will not be long-wearing and you are guaranteed to have to reapply the paint again.
Depending on how strong or bold the previous color is, it may be necessary to apply more than one coat of primer. However, it is not necessary to over apply the primer with so many coats. As long as the primer applies uniformly over the previous color, then one or two coats should be sufficient.