Dark to light/neutral
Just bought a house and all the walls on the first level are painted Dark reds, greens and blues. What tyoe of primer should I use and how many coats are needed
Really, you likely do not need any primer at all. Due to the amount of TiO2, pastels/whites actually hide better than darker paints. Now, if you are putting dark colors on top of a light surface, then you need a specialized hiding primer, grey-tinted to roughly match the darkness of the eventual topcoat.
That said, I have been priming all of my walls due to extensive drywall repairs and uncertainty about the quality of the existing topcoat. My "general purpose" primer has been Sherwin Williams PrepRite ProBlock Latex. Note that this is a light-duty stain blocking primer, so it actually does not hide that smoothly. After one coat, your walls will be a very blotchy white.
However, even with a scab-red undercoat, the one coat of primer, and two topcoats of Sherwin Williams SuperPaint, and everything was fine.
The real key to solid coverage of your topcoat is purchasing paint at or near the top of the product line from an actual paint store, as opposed to your local Big Box.
Only one coat primer
I'm presuming you're wanting to repaint those walls to an off white colour.
I think, if it wuz me, I'd spring for the $13 or $14 and have a quart of either Benjamin Moore or Pratt & Lambert's highest hiding MEDIUM tint base in a flat gloss tinted with black colourant to make a medium grey flat paint. That would give me the highest hiding paint I (personally) would know how to make. Then, see if you can get complete hide with one coat with that paint.
(That is, put LOTS of black colourant in a medium tint base to make a medium grey, not just a little bit of black colourant in a pastel base to make a light grey. Black is the highest hiding pigment in paint because it absorbs all incident light, so the more black pigment in your paint, the better it will hide.)
Paint an area of 2 square feet on a red, a blue and a green wall. Allow to dry. Then repaint one of those two square feet again and allow to dry. Then see if you can tell any difference between one coat of grey paint and two. If so, you're not getting complete hide in one coat.
If you can hide the underlying colour with a medium grey flat paint, then that's the way I'd go. Then, paint that grey the colour you want.
If you can't get complete hide in one coat, and you'd have to put on two coats of grey anyway, you probably wouldn't be saving any work or money by going to a grey paint because you'd still need to hide that grey undercoat with one or two applications of your top coat paint. In that case, I'd buy the paint I was intending to put on, and see if 3 or 4 coats will hide the underlying colour.
In a case like his, and as was mentioned previously, it's important to go with a high quality paint. The highest hiding white pigment used in house paints is titanium dioxide, which is relatively expensive compared to other white pigments, so if you want high hide in a paint, you need lots of titanium dioxide in it, and no paint company is going to give it away free. The more they put in the can, the more you're gonna have to pay for that can. So, spend more on your paint this time, and hopefully save on labour.
I agree with the previous comment that a good paint would hide better than a primer because of the amount of TiO2.
Titanium dioxide scatters light well, but it's not great at absorbing light like black is. To get good hide in paint, you need to be able to scatter incident light well and also have pigments in your paint that will absorb that scattered light. See if you can find a colour that calls for a fair bit of black or raw umber in it's tint formula that you like. Both of those colourants absorb light well, and adding them to a high quality white paint with lots of titanium dioxide in it will very much improve it's ability to hide an underlying colour.
PS: Post again if you want to know why titanium dioxide provides for better hide in paint than other white pigments like zinc oxide.
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