When does a panel need cradling?

Cradle thin panels (around 6mm and under, e.g. hardboard or 6mm MDF/ply) — they warp readily as paint and ground dry on one face. Larger panels need bracing regardless of thickness: as a working rule a panel much over 300mm (12in) on a side benefits from a perimeter frame, and panels well over that size also want interior cross-braces. The defaults here flag panels 6mm or thinner, or with a longest side over 600mm; adjust both thresholds under Advanced if you prefer the more cautious 300mm trigger used by Golden’s Just Paint guidance.

How are the cross-braces spaced?

Cross-bracing is applied in both directions (horizontal and vertical) at roughly 300mm (12in) centres on panels that need it — this is the spacing recommended in Golden Artist Colors’ Just Paint article on plywood substrates. This calculator counts the interior braces needed along each side at your chosen spacing, on top of the perimeter frame. Typical brace stock is nominal 1×2in (about 50×100mm), or rip your own 18×100mm strips from 18mm ply, which resists warping better than softwood batten.

What wood is best for painting panels?

Birch plywood is the preferred choice — stable, smooth and warp-resistant. MDF gives a flat face but is heavy and reacts to moisture, so seal it well. Avoid chipboard and OSB, which have coarse, unstable surfaces.

Do I need to seal the back of a wood panel?

Yes — seal the back and edges as well as the face, especially for oil. Coating only one side lets the panel take up moisture unevenly and cup; an equal seal (gesso, acrylic medium or size) on both faces and all edges keeps it flat.

ArtCalcs

Panel Size Calculator

Work out wood panel dimensions and cradling requirements for rigid painting surfaces.

Panel Width

The panel face edge-to-edge; default unit mm (switch to cm/in on the right).

Panel Height

The other panel face edge; longest of the two sides drives the size-based cradling trigger.

ThicknessBoard thickness. 3mm hardboard and 6mm MDF/ply are thin and warp-prone — they want cradling; 12–18mm is far more rigid.

How We Calculate This

Panel area = width × height (converted to m²). Cradling is flagged when the board is thin (6mm or less by default) or the longest side exceeds the size threshold (600mm by default) — both adjustable under Advanced.

When bracing is needed, the panel gets a perimeter frame plus interior cross-braces in both directions. Interior braces along each side = ceil(side ÷ spacing) − 1, with the default ~300mm (12in) spacing taken from Golden Artist Colors’ Just Paint guidance on plywood substrates. The 600mm and 300mm figures are working rules of thumb, not a single published standard — increase them for thicker stock or reduce them for thin panels you want to play safe with.

Frequently Asked Questions

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Last updated: June 2026

All calculations are estimates. Always verify quantities before purchasing materials.