Strange Paint Reaction on joints

any new surface ( plaster, plasterboard, mastic) need one or two coates of mist coat before painting. If the painter didn"t do it and start painting with the colour that is the result

Hi there, first able for new plaster you need sufficient time to dry before painting. Then you must prime it!!! before painting.As far as painting over mastic …. Most of them are not paintable so it has to go as last finish I’ve paint.
Buy yourself Zinsser primer spot blocker, remove mastic, prime it 2 coats, caulk/ patch if need it, paint 2 coats and then seal it if need it

Possibly a chemical reaction using mastic that shouldn’t be painted same as the wall might not of been prepared properly before painting this is a tricky one with decorating you need to be looking at it

Sand paint off and apply pva and water mixture. Allowed to dry and then paint over the top of it

Hello. I wasn't sure exactly what the issue may be but I pulled this off goggle since it may help //

Two things cause this: You have inadequate or poorly installed insulation, or you have an air leak on the outside wall surface that's letting cold air into the wall cavity. Most likely those shadow lines were there before you painted, and they became more apparent with the new paint.

Without having a closer look it sounds like it's PVA glue that has run down from when the plaster out was applying the PVA glue to the walls you will have to put some sort of stain block on bin zinsser He's a great choice initial block out the dark lines and then apply a couple of coats of your chosen top coat Hope this helps.
Chris
Decor. 27

Put masi en filer rn paint

First on new plaster should apply 1 coat to isolate the surface and after 2 coats off paint ,second,never paint the mastic.

it wasn't allowed to dry properly or he didn't tape the joints and you can see the joints

May he use chaulk at the joints , ????

Maybe strip everything back and prime the walls, on new and old plaster, before so use a degreaser on the walls to get rid of anything silicone based that may have gone into the plaster or on the wall, then prime.

Leave and dry properly and if is necessary to put another coat of easy filler and then painting

Paint dont adhere on Mastic. If the joints have mastic , try an undercoat binder...

The darker lines along joints where new plaster or mastic were applied could be due to moisture, uneven paint absorption, or surface preparation issues. It's essential to ensure proper drying, use compatible paint, and have the surfaces prepared correctly.

Well this does not happen and ot would be better to re coat it and sand the walls.

It sounds like you may be experiencing a phenomenon known as "joint shadowing" or "joint banding." This occurs when there is a difference in porosity or texture between the joints and the surrounding areas, causing the paint to dry differently and result in darker lines along the joints.

Here are some possible explanations and solutions:

1. **Difference in Absorbency:** New plaster or mastic may have a different absorbency compared to the surrounding surfaces, causing the paint to dry unevenly and appear darker along the joints. This can be exacerbated if the joints were not properly primed before painting.

- Solution: Ensure that all surfaces, including joints, are properly primed before painting to create a uniform base and minimize differences in absorbency. Use a high-quality primer suitable for the type of surface being painted.

2. **Moisture Content:** Differences in moisture content between the joints and surrounding areas can also cause joint shadowing. If the joints were not fully dry before painting, or if there is residual moisture from the plaster or mastic, it can affect the drying of the paint.

- Solution: Allow the joints to fully dry before painting, and ensure that the area is well-ventilated to facilitate drying. Avoid painting in high humidity conditions, as this can prolong drying times and exacerbate joint shadowing.

3. **Application Technique:** Uneven application of paint, such as applying too much paint or using inconsistent brush or roller techniques, can contribute to joint shadowing. This can result in differences in paint thickness and texture along the joints.

- Solution: Apply paint evenly and consistently across the entire surface, paying extra attention to joints and areas with new plaster or mastic. Use the appropriate tools and techniques for the type of surface being painted to achieve a smooth and uniform finish.

4. **Type of Paint:** Certain types of paint, particularly darker or more pigmented colors, may be more prone to joint shadowing than others. This is because darker colors tend to highlight imperfections and differences in texture more prominently.

- Solution: Consider using a lighter-colored paint or one with a lower sheen level, as these can help to minimize the appearance of joint shadowing. Alternatively, choose a paint specifically formulated to hide imperfections and provide a more uniform finish.

If the builder is planning to come back and address the issue, they may need to take steps such as re-priming the affected areas, applying additional coats of paint, or using specialized techniques to achieve a more uniform finish. Communicate your concerns to the builder and work together to find a solution that resolves the issue to your satisfaction.