For an alkyd stain blocking primer for exterior wood, we like Cabot Problem Solver Exterior Primer. It seals heavy bleed from redwood or other bleeding woods. However, we usually prefer Acrylic primer and finish for most exterior wood. Premium exterior acrylic primers will block bleeding stains (2 coats required, 24 hrs apart). The advantage of the Acrylic primer is long term flexibility, and some breathability. Alkyd primers typically are non-breathable, non-flexible, and continually harden as they age.
This is a much debated question, but we feel strongly that a premium acrylic primer like Coronado 410-11 is best as a first coat on most exterior wood. Much of the new wood used in new construction is high in moisture content, and the acrylic primer is much more tolerant of that moisture. Acrylic primers give much better surface gripping power if wood grain is tight, as is usually the case with exotic hardwoods. If bleeding occurs, either apply a second coat of acrylic primer, or if bleeding is severe, you could then consider a coat of the alkyd product. We just prefer the safety, ease, and durability of Acrylic products for most projects.
You must evaluate your specific project requirements when choosing your primer.