Here's what Hardie has to say about it:
AN EXCLUSIVE PRIMER FOR AN EXCEPTIONAL PRODUCT:
No matter what James Hardie siding product you choose, applying a top quality primer is the first critical step to ensure lasting performance and beauty. That’s why James Hardie developed PrimePlus® sealer and primer - the first factory applied primer designed specifically for fiber cement siding products.
FINISHING JAMES HARDIE SIDING PRODUCTS:
For best results with our exclusive PrimePlus® factory priming system, use high quality (premium), exterior grade topcoats. Paint PrimePlus® Primer within 180 days of installation. For best results with unprimed Hardie siding products, prime with high quality, exterior grade primers and finish with high quality (premium), exterior grade topcoats. Use primers and topcoats that are designed and recommended for cement based building materials (fiber-cement, masonry, brick or stucco). Paint unprimed Hardie siding products within 90 days of installation.
Note - The use of oil based paints on unprimed fiber cement could result in increased surface roughness, loss of adhesion, cracking or excessive chalking. James Hardie does not recommend the use of oil based paints over unprimed fiber cement siding products. Stains containing linseed oil are specifically designed for wood and may not be suitable for fiber cement siding products, primed or unprimed.
Note - Semi-transparent stains can vary in uniformity of appearance depending on method of application & conditions and will require a high level of skill & craftsmanship to achieve a uniform appearance. Clear coats have not proven durable in exterior exposure and James Hardie considers them a maintenance item that may require application of a refurbishing sealer at regular intervals. James Hardie does not warrant the appearance or durability of semitransparent stains or clear coats.