# Plasterboard and damp: water-repellent boards, joints and how to paint

> What a water-repellent board does and does not do, how joints are finished (Q1 to Q4), primer before paint and when it is better to replace the board than paint it.

- Page: https://paredecerta.pt/en/guides/plasterboard-damp-and-painting
- Language: English
- Updated: 2026-11-04

## Summary

- A water-repellent board (H1) absorbs less water at the surface but is not waterproof. Knauf rates it up to 80% permanent humidity.
- For showers and direct splashes, Knauf presents a cement board (Aquapanel Indoor).
- To paint, joints get tape, filler and sanding (levels Q1 to Q4) and the surface gets a coat of primer.
- A wet, soft or mouldy board is not fixed with paint: cause first, then decide whether to replace the board.

Plasterboard walls and ceilings (in Portugal often called "pladur", which is a brand) are common in new and renovated homes. Paper and gypsum do not like water. This guide explains what "water-repellent" boards are, what they do not do, how a board is prepared for paint, and when it is better to rebuild the wall than to paint it.

*Updated 3 November 2026. General information, not a substitute for an assessment by a technician.*

## The essentials

- A **water-repellent (type H1)** board absorbs less water at the surface. It is **not waterproof** and is not a shower lining.
- Knauf rates its H1 board for areas where permanent humidity does not exceed 80%. For showers, Knauf presents a different board, made of cement (Aquapanel).
- A board that is **soaked or mouldy** cannot be cured with paint. The US EPA says porous materials with mould may have to be thrown away.
- To paint a new board, the joints need **tape, filler and sanding**, and the whole surface needs **a coat of primer** before paint.
- Water stain on a plasterboard ceiling or wall: **find and fix the leak first**. Then check whether the board is still firm.

## Why damp affects plasterboard

A board is a gypsum core between two sheets of paper. The EPA says mould can grow on the back of drywall, wallpaper or panelling, and that a musty smell with no visible source can point to hidden mould. It also says porous materials, such as ceiling tiles, "may have to be thrown away" if they get mouldy.

With condensation, the board itself is rarely the cause. It is usually the cold wall behind it, or poor ventilation (see [condensation in the bedroom and the home](https://paredecerta.pt/en/guides/bedroom-condensation-ventilation)). For mould itself, see [mould on walls](https://paredecerta.pt/en/guides/mould-on-walls).

## Which boards exist for damp areas

We read the product pages of two manufacturers. The figures below are theirs, not ours.

| Board | What the manufacturer says |
|---|---|
| Knauf Hidrófuga H1 | Type H1 (EN 520), green paper. For areas with permanent humidity up to 80%, such as bathrooms and kitchens. In contact with water, about 48 hours before weight rises and strength drops. |
| Placo PPM (Saint-Gobain) | H1 (EN 520), for "interior areas of medium humidity", kitchens and bathrooms. Water absorption in the technical table: ≤ 5%. |
| Glasroc H (Saint-Gobain) | Type GM-H1, glass-fibre reinforced, for very humid areas. The page lists "high resistance to humidity, fungal growth and mould". |
| Knauf Aquapanel Indoor | Cement board, "100% resistant to water and mould". For showers, saunas and direct splashes. Takes ceramic tiles. |

Saint-Gobain also lists "Hydrostil" metal profiles and screws made for damp areas. The product pages we read contain no warnings about painting these boards.

For showers and direct splashes, Knauf's page presents the Aquapanel Indoor cement board, not the H1. That choice belongs to the building work, not the painting: ask the installer which board is behind the wall.

## How the board is prepared and painted

A document by Eurogypsum and the European federation of drywall contractors describes four joint finishing levels, before painting:

| Level | What it includes | What it suits (per the document) |
|---|---|---|
| Q1 | Tape and filler in joints, cover screws | Places with no decorative demands, or under tiles |
| Q2 | Q1 plus a second coat to smooth the transition | Normal finish of walls and ceilings; matt paints with medium texture |
| Q3 | Q2 plus a wider third coat and smoothing the surface | Fine, matt paints |
| Q4 | Q2 plus a thin skim over the whole surface | Medium-gloss paints and varnishes |

The same text says that if the project states no level, Q2 applies, which can still show contrasts in raking light, and that each coat must dry for the time the manufacturer gives. Paint goes on only once the filler is fully dry, and **a coat of primer over the whole surface** goes on first, repairs included. If you want a smooth wall under raking light, ask the painter or installer for Q3 or Q4 and put it in the [quote](https://paredecerta.pt/en/guides/compare-painting-quotes).

For paint, we read the data sheet of Dyruplad 2in1 (Dyrup, May 2019 edition), made for plasterboard and usable as primer and topcoat: surfaces well dried and dust-free; repairs with plaster filler; 2 to 3 coats; recoat after 4 hours at 23 °C and humidity below 60%; do not apply above 80% relative humidity, below 10 °C or above 30 °C. It is one example, not a recommendation. Follow the data sheet of the product you buy. For quantities, use the [calculator](https://paredecerta.pt/en#calc) and the guide [how much paint do I need](https://paredecerta.pt/en/guides/how-much-paint-do-i-need).

## Paint it or rebuild it?

We found no official rule on when a wet board should be replaced. What the sources allow us to say:

1. **Cause first.** A leak, infiltration or condensation keeps wetting the board if not fixed. See [painting after a leak is fixed](https://paredecerta.pt/en/guides/painting-after-a-leak-is-fixed) and [painting ceilings: technique and stains](https://paredecerta.pt/en/guides/painting-ceilings-technique-stains).
2. **Dry stain, firm board, no mould, no smell:** an isolating primer and paint, following the product's data sheet, fix the look (see the ceiling guide above).
3. **Soft, blistered, crumbling board, or mould or a musty smell:** the EPA points to removing the affected porous material. Do not paint over it. Talk to a professional, who should open the area and see what is behind.
4. **Large area or suspected hidden mould:** the EPA recommends calling an experienced professional.

## What we could not confirm

- **The exact drying time of a wet board.** We found no source. No manufacturer we read gives a "dry in X days" rule.
- **Which paint and primer each manufacturer requires for its boards.** The Knauf and Saint-Gobain product pages carry no painting instructions. We saw only the European document and one paint data sheet.
- **Whether an H1 board prevents mould.** The pages speak of lower absorption; only Glasroc H and Aquapanel mention fungi or mould.
- **Prices** of boards and labour. We used none.
- **Siniat and other makers in Portugal.** We found no Portuguese page.

## Sources (read on 3 November 2026)

- Knauf Portugal, [Hidrófuga H1](https://tools.knauf.pt/solucoes/placas/gesso-laminado/impregnada-h1.html): classification, humidity limit, behaviour in contact with water.
- Knauf Portugal, [Aquapanel Indoor](https://tools.knauf.pt/solucoes/placas/cimento/aquapanel-indoor-2.html): uses, mould resistance, finishing.
- Saint-Gobain (Placo), [PLACO PPM](https://www.saint-gobain.pt/Produtos/placo/placo-ppm), [Glasroc H](https://www.saint-gobain.pt/produtos/placo/glasroc-h) and [PLACO PR HYDRO](https://www.saint-gobain.pt/produtos/placo/placo-pr-hydro) (joint compound): classification, absorption, uses.
- Eurogypsum and UEEP, [Classification of surface finish quality levels](https://gyptec.eu/documentos/EUROGYPSUMFINSHINGP.pdf) (Portuguese copy, 12 pages, read in full).
- Dyrup, [Dyruplad 2in1, technical-commercial information](https://dyrup.pt/kentico/getmedia/82adaf30-2bfd-405e-b283-b69bedb4cf4b/ficha-tecnica_Dyruplad.pdf), May 2019 edition (read in full).
- U.S. EPA, [A Brief Guide to Mold, Moisture and Your Home](https://www.epa.gov/mold/brief-guide-mold-moisture-and-your-home) (hidden mould) and [Mold Cleanup in Your Home](https://www.epa.gov/mold/mold-cleanup-your-home) (porous materials, professional help).

## FAQ

### Does a water-repellent board stand up to a shower?

Knauf rates the H1 for permanent humidity up to 80%, such as bathrooms. For showers and direct splashes it presents the Aquapanel Indoor cement board.

### Do I need primer before painting plasterboard?

The Eurogypsum and UEEP document says to apply a coat of primer over the whole surface, repairs included, once the filler is dry.

### Can I paint over a water stain on a plasterboard ceiling?

Only after the leak is fixed and if the board is firm, with no mould or musty smell. With mould, the EPA says porous material may have to be removed.

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Source: https://paredecerta.pt/en/guides/plasterboard-damp-and-painting. Independent information, not legal or tax advice.