Install-Package RichardLawley.WebApi.OrderedFilters
Install-Package RichardLawley.WebApi.FluentValidation

// Change the Filter Provider to one which respects ordering
config.Services.Replace(typeof(System.Web.Http.Filters.IFilterProvider), new OrderedFilterProvider());

// Filters for FluentValidation
config.Filters.Add(new FluentValidationActionFilter());     // Runs FluentValidation

// Optional - prevent validation errors reaching controller (no need to check ModelState.IsValid)
config.Filters.Add(new ValidationActionFilter());           // Prevents validation errors reaching controller

// Configure the FluentValidation integration
builder.RegisterType<ScopedValidatorFactory>().As<IScopedValidatorFactory>();
builder.RegisterType<FluentValidatorProvider>().As<IFluentValidatorProvider>().SingleInstance();

builder.RegisterType<MyValidator>().As<IValidator<MyType>>();