You want to paint a hot-water radiator, an exposed pipe or an iron railing and are not sure which paint to buy. Wall paint will not do: on hot, rusty metal you need a purpose-made enamel and the right preparation. This guide covers the order of work, the temperature the paint must withstand and when a primer is needed, using figures from the data sheets we read.
Updated on 4 November 2026. General information, not a substitute for a technician’s assessment. Always follow the data sheet of the paint you buy.
The essentials
- Paint with the radiator switched off and cold. The Isaval and Montana data sheets say so.
- Loose rust and grease come off first. Without that, paint peels, whatever the brand.
- Check on the tin what temperature the paint withstands. The radiator enamels we read state 80 ºC continuous, 100 ºC or higher, depending on the product.
- After painting, leave the radiator off. One data sheet asks for 72 hours before switching it on and 12 hours of ventilation after.
- In an old house, treat the old paint as if it contained lead before sanding: see old paint and lead.
Which paint for a radiator
Look for an enamel sold for radiators. The sheets we read cover three types.
| Type | Example read | Stated temperature | Notes from the sheet |
|---|---|---|---|
| Synthetic enamel (solvent) | Isaval, Isalnox Esmalte radiadores, version of 4 May 2026 | 80 ºC continuous, 140 ºC peaks | Recoat after 4 hours, 2 coats, 12 m² per litre |
| Alkyd enamel in a spray can | Montana Colors, Pintura radiadores (code 280421 on the sheet) | 100 ºC | Two thin coats, full dry 24 to 48 hours |
| Water-based acrylic enamel | San Marco, Unimarc Smalto per Radiatori, 23 September 2024 | The sheet gives no figure, it says it resists high temperatures | Odourless, 1 to 2 coats, recoat after 6 hours |
San Marco describes its water-based enamel as odourless, which helps in an occupied home, and states a maximum of 130 g/l VOC. To read that figure on a label, see paint, VOC and indoor air.
“High-temperature” paints are made for chimneys and ovens. Isaval’s water-based black matt one withstands 450 ºC and reaches that resistance only after 48 hours. The Barbot spray sheet requires two days of curing and gradual heating, and gives different temperature figures in different places. For a domestic radiator, choose an enamel made for radiators.
Enamel and emulsion are not the same thing: see paint types.
Preparation: what decides whether the paint lasts
1. Switch off and let it cool
Isaval’s sheet says to paint with the radiator off and cold, and not to use the product on electric or oil-filled radiators. Montana’s says not to apply to equipment that is switched on.
2. Clean and remove rust
The sheets ask for a clean, dry surface free of dust, grease and salts. Isaval asks for rust to be scraped and brushed off, by hand or machine, until the surface is free of oxide. For iron parts, Dyrup recommends cleaning rust to grade St2 or Sa 2½ of ISO 8501 and removing poorly adhered paint.
If the old paint is sound and well bonded, Isaval accepts painting over it. If it flakes, take it off. San Marco says to sand the whole surface to roughen it and degrease with thinner.
3. Convert rust that will not come off completely
A rust converter turns adhered rust into a film the paint can grip. Barbot’s Conversor de Ferrugem 1042 sheet (13 December 2024, revision 3) says: steel and iron, brush off loose rust first, apply by brush undiluted, 1 coat, 8 m² per litre, 24 hours between coats, clean tools with water, wear gloves and goggles. Its stated exposure is “exterior”, so ask the maker whether it suits an indoor radiator.
4. Primer
- Isaval says its radiator enamel needs no anti-rust primer on rust-free iron and steel, “except in aggressive environments”, where it recommends one. On non-ferrous metals it asks for 1 or 2 coats of primer.
- San Marco asks for its metal primer on iron and, on aluminium, zinc and thin sheet, on the areas that had rust.
- Dyrup’s iron primer (ref. 1490, December 2017) is alkyd, grey or brown: 1 to 2 coats, touch dry in 30 minutes, recoat after 2 to 3 hours, fully dry in 8 hours, 12 m² per litre.
Simple rule: a new, smooth radiator without rust may skip primer, according to Isaval. A rusty pipe or radiator, or an aluminium one, deserves primer.
How to paint, step by step
- Switch the heating off and wait until the radiator is fully cold. Protect floor and wall with plastic and masking tape.
- Brush off rust and sand flaking paint. Wipe away the dust.
- Degrease and let dry. Apply the converter where rust remains, if needed.
- Apply primer if the sheet asks for it. Respect the recoat time.
- Apply the enamel in thin coats, top to bottom, with a small brush or foam roller. Montana says two thin coats beat one thick coat.
- Respect the time between coats: 4 hours (Isaval), 6 hours (San Marco). For the Montana spray, recoat after 30 minutes and before 2 hours.
- Let it cure. Isaval asks for the radiator to stay off, at room temperature, for at least 72 hours before switching on, then good ventilation of the room for 12 hours.
Working conditions
The sheets give different limits, so read yours:
| Sheet | Air temperature | Relative humidity |
|---|---|---|
| Isaval, radiator enamel | Do not paint below 5 ºC | Maximum 80 % |
| San Marco, Unimarc | Between 8 ºC and 35 ºC (surface 5 to 35 ºC) | Below 75 % |
| Dyrup, iron primer | Between 10 ºC and 35 ºC | Maximum 80 % |
Isaval asks for the surface to be 2 to 3 ºC above the dew point. Cold metal beside a window with condensation does not meet that: see condensation in the bedroom.
Pipes, railings and other metal
The logic is the same: remove loose rust and grease, convert remaining rust, iron primer, then enamel. Isaval also lists its enamel for hot-water piping. Next to wood, see painting wood, doors and windows. To estimate litres, use the calculator, but an enamel’s coverage (12 m² per litre on one sheet) differs greatly from wall paint.
Safety
- Solvent products smell strongly. Dyrup asks for gloves tested to EN 374, work in a ventilated place or with extraction, and a respirator with an A/P2 filter.
- Keep spray cans away from a hot radiator: Montana’s sheet says the can itself resists heat only below 50 ºC.
- Sanding old paint can release lead dust. Read the lead guide before starting.
- Isaval says to keep the product away from children and not to bite painted surfaces.
What we could not confirm
- Your radiator’s working temperature. We found no official figure, and it depends on the installation. Ask the installer.
- A CIN, Robbialac or Dyrup data sheet for radiator enamel. We did not find one, so we used other brands.
- Prices. We give none because we found no dated source.
- Whether Barbot’s rust converter suits indoor use. The sheet states exterior exposure.
- The date of the Montana sheet. The file carries a code (280421) that may be a date, but we could not confirm it.
Sources (read on 4 November 2026)
- Pinturas Isaval, Isalnox Esmalte radiadores, data sheet 1996, version 4, 4 May 2026, read in full, in Spanish.
- Pinturas Isaval, Acquatex Altas temperaturas, data sheet 1140, version 3, 4 March 2025, read in full, in Portuguese.
- San Marco Group, Unimarc Smalto per Radiatori, data sheet, revision of 23 September 2024, read in full, in Spanish.
- Montana Colors, Pintura radiadores, data sheet, code 280421, read in full, in Spanish.
- Barbot, 1042 Conversor de ferrugem, data sheet, 13 December 2024, read in full, in Portuguese.
- Barbot, Alta Temperatura, data sheet, January 2017, read in full, in Portuguese.
- PPG Dyrup, Primário para ferro, ref. 1490, technical and commercial information, December 2017, read in full, in Portuguese.
- Lead in old paint: see the sources of our guide old paint and lead, not re-read today.